drm/amdgpu: change pointer of mqd_ptr & mqd_backup to void

vi_mqd is only used by VI family but mqd_ptr and mqd_backup is
common for all ASIC, so change the pointer to void.

Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Monk Liu <Monk.Liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>