drm/plane-helper: Fixup mismerge

I somehow manage to screw up applying Laurent's patch in eca93e28c256:
"drm: Check in setcrtc if the primary plane supports the fb pixel
format". It was a conflict with

commit 3461b30b3e171e16498f3d7bc59ab703aec475c8
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Mar 5 10:32:44 2015 +0100

    drm/plane-helper: unexport drm_primary_helper_create_plane

and I just didn't check that the solution from wiggle made sense.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: laurent.pinchart@ideasonboard.com
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: laurent.pinchart@ideasonboard.com
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>