drm/plane-helper: Test for plane disable earlier

drm_plane_helper_check_update() currently uses crtc before testing whether
we're disabling the plane (fb == NULL).  Move the fb test before the first crtc
usage so that crtc == NULL doesn't have to be handled by the caller.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>