drm-tests: add AR30/AB30 to null_platform_test

This CL adds AR30 and AB30 (*) to the list of formats supported by
null_platform_tests. These two formats cannot be scanned out, so
the opaque ones are chosen instead when importing into DRM (AddFB2).

This CL extends minimally the error string to also say if we
wanted modifiers or not.

(*) Full names [DRM,GBM]_FORMAT_A[RGB,BGR]2101010...
(Needs crrev.com/c/1925250 patched in).

BUg=chromium:949260
TEST=null_platform_tests -f AR30 (or AB30) on kohaku

Change-Id: Ic5475ee0e2fefcf50986a024822c1cf64c960080
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/drm-tests/+/1925247
Tested-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
2 files changed