drm/i915/gen11: Program the Y and UV plane for planar mode correctly, v3.

The UV plane is the master plane that does all color correction etc.
It needs to be programmed with the dimensions for color plane 1 (UV).

The Y plane just feeds the Y pixels to it. Program the scaler from the
master only, and set PLANE_CTL_YUV420_Y_PLANE on the slave plane.

Changes since v1:
- Make a common skl_program_plane, and use it for both plane updates.
Changes since v2:
- Make color_plane explicit, to clarify skl_update_plane(). (Ville)

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/20181018115134.9061-8-maarten.lankhorst@linux.intel.com