drm/tests: Do not use drm_fb_blit() in format-helper tests

Export additional helpers from the format-helper library and open-code
drm_fb_blit() in tests. Prepares for the removal of drm_fb_blit(). Only
sysfb drivers use drm_fb_blit(). The function will soon be removed from
format helpers and be refactored within sysfb helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250616083846.221396-2-tzimmermann@suse.de