drm/ssd130x: Fix atomic_check for disabled planes

The plane's atomic_check returns -EINVAL if the CRTC has not been
set. This is the case for disabled planes, for which atomic_check
should return 0. For disabled planes, it also omits the mandatory
call to drm_atomic_helper_check_plane_state().

Replace the test with the boiler-plate code that first invokes
drm_atomic_helper_check_plane_state() and then tests for the plane
to be visible. Return early for non-visible planes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: d51f9fbd ("drm/ssd130x: Store the HW buffer in the driver-private CRTC state")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009141018.11291-7-tzimmermann@suse.de