drm/radeon: implement wrapper for GET_SYSTEM_PARAMS

Use GET_SYSTEM_PARAMS for retrieving the configuration for the system
BIOS notifications.
v2: packed struct (Lee, Chun-Yi <jlee@suse.com>)
v3: fix enable with device specific command code

Signed-off-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>