drm/amdgpu: potential uninitialized variable in amdgpu_vce_ring_parse_cs()

We shifted some code around in commit 9cca0b8e ("drm/amdgpu: move
amdgpu_cs_sysvm_access_required into find_mapping") and now my static
checker complains that "r" might not be initialized at the end of the
function.  I've reviewed the code, and that seems possible, but it's
also possible I may have missed something.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org