drm/amdgpu: Fixing "Indirect register access for Navi12 sriov" for vega10

The NV12 and VEGA10 share the same interface W/RREG32_SOC15*,
the callback functions in these macros may not be defined,
so NULL pointer must be checked but not in
macro __WREG32_SOC15_RLC__, fixing the lock of NULL pointer check.

Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>