drm/amdgpu: Uninitialized variable in amdgpu_ttm_backend_bind()

My static checker complains that it's possible for "r" to be
uninitialized.  It used to be set to zero so this returns it to the old
behavior.

Fixes: 98a7f88c ("drm/amdgpu: bind BOs with GTT space allocated directly v2")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>