test/kms_async_flips: fix test abortion due to unreadable mod name

When looking for names of non-intel modifiers, igt_fb_modifier_name()
returns "?" which causes run_test_with_modifiers abortion and therefore
a regression to `async-flip-with-page-flip-events` test case, as below:

(kms_async_flips:9663) igt_core-CRITICAL: Invalid dynamic subtest name "pipe-A-eDP-1-?".
kms_async_flips: ../lib/igt_core.c:2296: igt_exit: Assertion `!test_with_subtests || skipped_one || succeeded_one || failed_one' failed.
Received signal SIGABRT.
Stack trace:
 #0 [fatal_sig_handler+0x17b]
 #1 [__sigaction+0x40]
 #2 [pthread_key_delete+0x14c]
 #3 [gsignal+0x12]
 #4 [abort+0xd3]
 #5 [<unknown>+0xd3]
 #6 [__assert_fail+0x42]
 #7 [igt_exit+0x1ca]
 #8 [__igt_run_dynamic_subtest+0x122]
 #9 [run_test_with_modifiers.constprop.0+0x1c7]
 #10 [__igt_unique____real_main635+0x56b]
 #11 [main+0x27]
 #12 [__libc_init_first+0x8a]
 #13 [__libc_start_main+0x85]
 #14 [_start+0x21]
Aborted

Replacing "?" with "unknown" solves the issue and async flip tests run
again.

Fixes: 7fa7602c ("tests/kms_async_flips: Test all modifiers")
Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>