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

commit c4aa8dff6091cc9536aeb255e544b0b4ba29faf4 upstream.

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>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>