drm/i915: Fix plane YCbCr->RGB conversion for GLK

drm/i915: Fix plane YCbCr->RGB conversion for GLK

On GLK the plane CSC controls moved into the COLOR_CTL register.
Update the code to progam the YCbCr->RGB CSC mode correctly when
faced with an YCbCr framebuffer.

The spec is rather confusing as it calls the mode "YUV601 to RGB709".
I'm going to assume that just means it's going to use the YCbCr->RGB
matrix as specified in BT.601 and doesn't actually change the gamut.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214192327.3250-6-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>