drm: Add drm_format_num_planes() utility function

This function returns the number of planes used by a specific pixel
format.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>