drm/radeon: allow MIP_ADDRESS=0 for MSAA textures on Evergreen

MIP_ADDRESS should point to the resolved FMASK for an MSAA texture.
Setting MIP_ADDRESS to 0 means the FMASK pointer is invalid (the GPU
won't read the memory then).

The userspace has to set MIP_ADDRESS to 0 and *not* emit any relocation
for it.

Signed-off-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org