drm/amdgpu: Potential uninitialized variable in amdgpu_vm_update_directories()

commit 78aa02c713fcf19e9bc8511ab61a5fd6c877cc01 upstream.

After commit ea09729c9302 ("drm/amdgpu: rework page directory filling
v2") then it becomes a lot harder to verify that "r" is initialized.  My
static checker complains and so I've reviewed the code.  It does look
like it might be buggy... Anyway, it doesn't hurt to set "r" to zero
at the start.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>