drm/amd/display: Expose target backlight level

[Why]
DM may want to understand any backlight optimizations
applied, so DM needs a way to query from the HW both
the real current backlight, which may be value during
transition.
And also target backlight, which may be after some
backlight optimizations applied.

[How]
Add interface to query current and target backlight levels
Target level may indicate backlight level after backlight
optimization and reductions are applied.

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>