drm/amd/display: Increase Backlight Gain Step Size

[Why]
Some backlight tests fail due to backlight settling
taking too long. This happens because the step
size used to change backlight levels is too small.

[How]
1. Change the size of the backlight gain step size
2. Change how DMCU firmware gets the step size value
   so that it is passed in by driver during DMCU initn

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>