drm/i915: Make sure plane dims are correct for UV CCS planes

As intel_fb_plane_get_subsampling() returns the subsampling factor wrt.
its main plane, for a CCS plane we need to apply both the main and the
CCS plane's subsampling factor on the FB's dimensions to get the CCS
plane's dimensions.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191231233756.18753-5-imre.deak@intel.com