drm/cirrus: Remove format test from cirrus_fb_create()

The DRM core implements a format check when setting a framebuffer
for a plane. [1] Remove the unnecessary test from cirrus_fb_create().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://elixir.bootlin.com/linux/v6.1/source/drivers/gpu/drm/drm_atomic.c#L629 # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-12-tzimmermann@suse.de