drm/i915/gen11: Handle watermarks correctly for separate Y/UV planes, v2.

Skylake style watermarks program the UV parameters into wm->uv_wm,
and have a separate DDB allocation for UV blocks into the same plane.

Gen11 watermarks have a separate plane for Y and UV, with separate
mechanisms. The simplest way to make it work is to keep the current
way of programming watermarks and calculate the Y and UV plane
watermarks from the master plane.

Changes since v1:
- Constify crtc_state where possible.
- Make separate paths for planar formats in skl_build_pipe_wm() (Matt)
- Make separate paths for calculating total data rate. (Matt)
- Make sure UV watermarks are unused on gen11+ by adding a WARN. (Matt)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018115134.9061-5-maarten.lankhorst@linux.intel.com