drm/nouveau: uvmm: remove incorrect calls to mas_unlock()

Remove incorrect calls to mas_unlock() in the unwind path of
__nouveau_uvma_region_insert(). The region maple tree uses an external
lock instead, namely the global uvmm lock.

Fixes: b88baab8 ("drm/nouveau: implement new VM_BIND uAPI")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230807163238.2091-5-dakr@redhat.com