For the vram_start is 0 case, the gart range will be from 0x0000FFFF00000000
to 0x0000FFFF1FFFFFFF, which will cause the engine hang.

So to avoid the hole, limit the max mc address to AMDGPU_GMC_HOLE_START.:wq
Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>