Extract the meat of skl_compute_plane_wm_params() into a lower
level helper that doesn't depend on the plane state. We'll
reuse this for the cursor ddb allocation calculations.

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-4-ville.syrjala@linux.intel.comReviewed-by: Matt Roper <matthew.d.roper@intel.com>