drm/amdgpu/gmc6: Place gart at low address range

drm/amdgpu/gmc6: Place gart at low address range

Instead of using a best-fit algorithm to determine which part
of the VMID 0 address space to use for GART, always use the low
address range.

A subsequent commit will use this to map the VCPU BO in GART
for the VCE1 IP block.

Split this into	a separate patch to make it easier to bisect,
in case	there are any errors in	the future.

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