drm/i915: Extract skl_crtc_calc_dbuf_bw()

Extract the dbuf slice data_rate calculation into a small
helper. Should make it a bit easier to handle the different
color planes of planar formats correctly.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220118092354.11631-7-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>