drm/amdgpu/vcn: Add vcn firmware log

vcn fwlog is for debugging purpose only,
by default, it is disabled.

Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>