drm/amd/display: Expand dc to use 16.16 bit backlight

[Why] We want to increase precision for backlight setting.
But DC interface takes 8 bit backlight level value only.

[How] DMCU already takes 16 bit backlight level.
Expand the DC interface to take 16.16 bit value.
Max 32 bit backlight value (0xFFFF) will represent
max backlight (100%)

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