drm/i915: Enable YUV to RGB for Gen10 in Plane Ctrl Reg

If the fb format is YUV, enable the plane CSC mode bits
for the conversion.

v2: Addressed review comments from Shashank Sharma
Alignment issue fixed in i915_reg.h

v3: Adding Reviewed By from Shashank Sharma

v4: Rebased the patch. As part of rebasing, re-using
the color series defines which are already merged.
plane_state->base.color_encoding might not be set for
NV12. For now, just using PLANE_COLOR_CSC_MODE_YUV709_TO_RGB709
in glk_plane_color_ctl if format is NV12.

v5: Added reviewed by from Juha-Pekka Heikkila

v6: Rebased the series

Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-12-git-send-email-vidya.srinivas@intel.com