drm/amdgpu/userq: fix error handling of invalid doorbell

If the doorbell is invalid, be sure to set the r to an error
state so the function returns an error.

Reviewed-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 7e2a5b0a)
Cc: stable@vger.kernel.org