lib/igt_device_scan: fix igt_devices_free

igt_devices_scan returns with empty list if it is called after
anyone calls igt_devices_free. Fixed this with clearing
igt_devs.devs_scanned inside igt_devices_free. Also while at
this, fix memory leak in it.

Remove redudant code from igt_devices_scan and replace it with
call to igt_devices_free

v2: optimize code in igt_devices_scan (Zbigniew)
v3: update commit message (Janusz)
v4: [Kamil] re-worded commit message and commit description

Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Acked-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>