drm/i915: Pass the entire skl_plane_wm to skl_compute_transition_wm()

drm/i915: Pass the entire skl_plane_wm to skl_compute_transition_wm()

We have to pass both level 0 watermark struct and the transition
watermark struct to skl_compute_transition_wm(). Make life less
confusing by just passing the entire plane watermark struct that
contains both aforementioned structures.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-8-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>