drm/plane: Fix a couple of checkpatch warnings

Code should be indented using tabs rather than spaces (see CodingStyle)
and the canonical form to declare a constant static variable is using
"static const" rather than "const static". Fixes the following warnings
from checkpatch:

	$ scripts/checkpatch.pl -f drivers/gpu/drm/drm_plane_helper.c
	WARNING: storage class should be at the beginning of the declaration
	#40: FILE: drivers/gpu/drm/drm_plane_helper.c:40:
	+const static uint32_t safe_modeset_formats[] = {

	WARNING: please, no spaces at the start of a line
	#41: FILE: drivers/gpu/drm/drm_plane_helper.c:41:
	+       DRM_FORMAT_XRGB8888,$

	WARNING: please, no spaces at the start of a line
	#42: FILE: drivers/gpu/drm/drm_plane_helper.c:42:
	+       DRM_FORMAT_ARGB8888,$

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