drm/amdgpu: don't map BO in reserved region

2MB area is reserved at top inside VM.

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