drm/amdgpu: fix VA hole handling on Vega10 v3

Similar to the CPU address space the VA on Vega10 has a hole in it.

v2: use dev_dbg instead of dev_err
v3: add some more comments to explain how the hw works

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