drm/i915: Don't check uv_wm in skl_plane_wm_equals()

drm/i915: Don't check uv_wm in skl_plane_wm_equals()

The hardware never sees the uv_wm values (apart from
uv_wm.min_ddb_alloc affecting the ddb allocation). Thus there
is no point in comparing uv_wm to determine if we need to
reprogram the watermark registers. So let's check only the
rgb/y watermark in skl_plane_wm_equals(). But let's leave
a comment behind so that the next person reading this doesn't
get as confused as I did when I added this check.

If the ddb allocation ends up changing due to uv_wm
skl_ddb_add_affected_planes() takes care of adding the plane
to the state.

TODO: we should perhaps just eliminate uv_wm from the state
and simply track the min_ddb_alloc for uv instead.

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/20200228203552.30273-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>