drm: Move drm_format_num_planes() to drm_crtc.c

There will be a need for this function in drm_crtc.c later. This
avoids making drm_crtc.c depend on drm_crtc_helper.c.

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