DRM core already checks in drm_atomic_plane_check() if the
pixelformat is valid. Hence we can collapse the default case
of the switch statement with the XRGB8888 case.

No functional change.
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>