drm/ssd130x: Use drm_atomic_get_new_plane_state()

The struct drm_plane .state shouldn't be accessed directly but instead the
drm_atomic_get_new_plane_state() helper function should be used.

This is based on a similar patch from Thomas Zimmermann for the simpledrm
driver. No functional changes.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220923083447.1679780-1-javierm@redhat.com