drm/amdgpu: implement VCE two instances support

VCE 3.0 has two indentical instances in the engine, they share
the same registers name in differrent memory block distinguished
by the grbm_gfx_index, we set to master instance after init, it
will dispatch task to slave instance. These two instances will
share the same firmware, but have their own stacks and heaps.

v2: add mutex for using grbm_gfx_index

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>