drm/amdgpu: fix amdgpu_cs_p1_user_fence

The offset is just 32bits here so this can potentially overflow if
somebody specifies a large value. Instead reduce the size to calculate
the last possible offset.

The error handling path incorrectly drops the reference to the user
fence BO resulting in potential reference count underflow.

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