Add CRTC background color property and methods to get & set it

Intel GEN9+ platforms support a programmable CRTC background color
below HW planes. The background color can be seen on areas not
covered by the planes. This CL adds the background color property
exposed by kernel, and the methods to get & set the property.

BUG=chromium:894259

Change-Id: I2f43eb91e76f8d7f22faa0c75de553a066f8fdee
Reviewed-on: https://chromium-review.googlesource.com/c/1278858
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Dongseong Hwang <dongseong.hwang@intel.com>
Auto-Submit: Wei Li <wei.c.li@intel.com>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624781}
9 files changed