drm/amd/display: initialize backlight_ramping_override to false

[Why]
Stack variable params.backlight_ramping_override is uninitialized, so it
contains junk data

[How]
Initialize the variable to false

Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>