drm/amd: Add abm level drm property

drm/amd: Add abm level drm property

Adaptive Backlight Management (ABM) is a feature
that reduces backlight level to save power, while
increasing pixel contrast and pixel luminance
to maintain readability and image quality.

ABM will adjust in response to the
pixel luminance of the displayed content.

ABM is made available as a drm property on eDP
monitors called "abm level", which ranges from 0 to 4.
When this property is set to 0, ABM is off.  Levels 1
to 4 represent different ranges of backlight reduction.
At higher levels both the backlight reduction and pixel
adjustment will be greater.

ABM requires DMCU firmware, which is currently available for
Raven ASICs only.  If the feature does not work, please
ensure your firmware is up to date.

v2:
Fix commit message, only attach property if DMCU loaded
v3:
Store ABM level in crtc state to accommodate dc
v4:
Fix ABM saving on dpms cycle

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>