drm/tests: Add new format conversion tests to better cover drm_fb_blit()

To fully cover drm_fb_blit(), add format conversion tests that are only
supported through drm_fb_blit().

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918-final-gsoc-v3-2-b999c042a4cc@riseup.net