drm: Install drm_fourcc.h

Userspace needs this header.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/drm/Kbuild

 header-y += drm.h
+header-y += drm_fourcc.h
 header-y += drm_mode.h
 header-y += drm_sarea.h
 header-y += i810_drm.h