drm/selftests: Add tests for drm_format_info* helpers

Add selftests for the following newly added functions:
 - drm_format_info_block_width
 - drm_format_info_block_height
 - drm_format_info_min_pitch

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101151051.1509-6-alexandru-cosmin.gheorghe@arm.com