drm/ttm: fix error handling in ttm_buffer_object_transfer

drm/ttm: fix error handling in ttm_buffer_object_transfer

Unlocking the resv object was missing in the error path, additionally to
that we should move over the resource only after the fence slot was
reserved.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Fixes: c8d4c18bfbc4a ("dma-buf/drivers: make reserving a shared slot mandatory v4")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20250616130726.22863-3-christian.koenig@amd.com