skl_compute_plane_wm() doesn't actually need the plane state. While
it would make logically sense to pass it, we shall need to reuse
skl_compute_plane_wm() to compute the minimum ddb allocation for
the cursor before the cursor may be enabled. Thus we can't rely
on the plane state. The alternative would be to duplicate a lot of
the wm calculations for the cursor ddb allocation case, which doens't
appeal to me.

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312205844.6339-3-ville.syrjala@linux.intel.comReviewed-by: Matt Roper <matthew.d.roper@intel.com>