drm/amdgpu: fix mask in GART location calculation

We need to mask the lower bits not the upper one.

Fixes: ec210e32 drm/amdgpu: put GART away from VRAM v2

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