drm/amd/display: gradually ramp ABM intensity

[Why]
Need driver to pass values of backlight ramp start and ramp reduction so
that intensity can be ramped down appropriately.

[How]
Using abm_parameters structure to get these values from driver.

Signed-off-by: Rizvi <syerizvi@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>