drm/i915: Add helpers to select correct ccs/aux planes

Using helpers instead of open coding this to select a CCS plane for a
main plane makes the code cleaner and less error-prone when the location
of CCS plane can be different based on the format (packed vs. YUV
semiplanar). The same applies to selecting an AUX plane which can be a
UV plane (for an uncompressed YUV semiplanar format), or a CCS plane.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kahola <mika.kahola@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>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191221120543.22816-5-imre.deak@intel.com