Plop in support for 10bit LUT on ilk/snb.

There is no split gamma mode on these platforms, so we have
to choose between degamma and gamma. That could be a runtime choice
but for now let's just advertize the gamma as having 1024 entries.
We'll also keep the ctm hidden for now.

v2: Don't use I915_WRITE_FW() yet
    Introduce bool has_ctm (Maarten)
    Call drm_crtc_enable_color_mgmt() uncoditionally (Maarten)
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401200231.2333-5-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>