drm/amdgpu/gmc: add a way to force a particular placement for GART

We normally place GART based on the location of VRAM and the
available address space around that, but provide an option
to force a particular location for hardware that needs it.

v2: Switch to passing the placement via parameter

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