drm/amdgpu: validate size and offset of user fence BO

We need to validate the offset to make sure that we don't write after the BO.

Additional to that a page should be enough and can make address space
handling much easier.

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>