tests/i915_suspend: Refresh device list after *-without-i915 subtests

If any of *-without-i915 subtests fails or skips for any reason, it may
leave the i915 module unloaded while keeping our device list populated
with initially collected data.  In a follow up igt_fixture section we then
try to reopen the device.  If the test has been executed with a device
filter specified, an attempt to open the device finds a matching entry
that belongs to the no longer existing device in that initially collected
device list, fails to stat() it, concludes that's because of the device
having been already open, and returns an error.  While that error,
triggered after subtests completion, doesn't affect results of the
subtest, reported by CI togethger with those results it is confusing to
users reviewing those reports.

Fix this issue by refreshing the potentially outdated device list before
continuing with drm_open_driver() if we've been called with a device
filter specified.

While being at it, add a comment that explains why we call
igt_devices_scan() from __igt_device_card_match() but don't force device
rescan, and emit a debug message if we fail in _is_already_opened() on
unsuccessful device stat().

v2: don't free the device list -- we can't tell if it has been populated,
    and igt_devices_free() fails if it hasn't,
  - commit message updated, description improved.

Subtest basic-s3-without-i915: FAIL (9.572s)
(i915_suspend:9050) drmtest-WARNING: card maching filter 0 is already opened
(i915_suspend:9050) drmtest-CRITICAL: Test abort in function drm_open_driver, file ../lib/drmtest.c:639:
(i915_suspend:9050) drmtest-CRITICAL: abort condition: fd < 0
(i915_suspend:9050) drmtest-CRITICAL: Last errno: 2, No such file or directory
(i915_suspend:9050) drmtest-CRITICAL: No known gpu found for chipset flags 0x1 (intel)
Test i915_suspend failed.
**** DEBUG ****
(i915_suspend:9050) drmtest-DEBUG: Looking for devices to open using filter 0: pci:vendor=intel,device=dg2
(i915_suspend:9050) drmtest-DEBUG: Filter matched /dev/dri/card0 | /dev/dri/renderD128
(i915_suspend:9050) drmtest-WARNING: card maching filter 0 is already opened
(i915_suspend:9050) drmtest-CRITICAL: Test abort in function drm_open_driver, file ../lib/drmtest.c:639:
(i915_suspend:9050) drmtest-CRITICAL: abort condition: fd < 0
(i915_suspend:9050) drmtest-CRITICAL: Last errno: 2, No such file or directory
(i915_suspend:9050) drmtest-CRITICAL: No known gpu found for chipset flags 0x1 (intel)
(i915_suspend:9050) igt_core-INFO: Stack trace:
(i915_suspend:9050) igt_core-INFO:   #0 ../lib/igt_core.c:2066 __igt_abort()
(i915_suspend:9050) igt_core-INFO:   #1 ../lib/drmtest.c:573 drm_open_driver()
(i915_suspend:9050) igt_core-INFO:   #2 ../tests/i915/i915_suspend.c:258 __igt_unique____real_main245()
(i915_suspend:9050) igt_core-INFO:   #3 ../tests/i915/i915_suspend.c:245 main()
(i915_suspend:9050) igt_core-INFO:   #4 ../sysdeps/nptl/libc_start_call_main.h:58 __libc_start_call_main()
(i915_suspend:9050) igt_core-INFO:   #5 ../csu/libc-start.c:128 __libc_start_main@@GLIBC_2.34()
(i915_suspend:9050) igt_core-INFO:   #6 [_start+0x2a]
****  END  ****

Fixes: f7aff600 ("tests/i915/i915_suspend: Disable d3cold_allowed for basic-s2idle-without-i915")
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Riana Tauro <riana.tauro@intel.com>
Link: https://patchwork.freedesktop.org/patch/522501/


Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>