There's a helper in drm_fourcc.h these days to check of we're dealing
with a two plane YUV format. Make use if it.

Also s/plane/color_plane/ in skl_plane_relative_data_rate() to reduce
the confusion.
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913193157.9556-2-ville.syrjala@linux.intel.com