drm/amdgpu/gmc6: use the vram location programmed by the vbios

This makes mc programming much simpler in future patches.

Since evergreen, the vbios has been programming the fb location
to the proper vram size.  The only reason to reprogram it would
be to change the location.

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