drm/amdgpu: check rlc_g firmware pointer is valid before using it

In SRIOV, rlc_g firmware is loaded by host, guest driver won't load it which will
cause the rlc_fw pointer is null

Signed-off-by: shaoyunl <shaoyun.liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>