Since David Herrmann's mmap vma manager rework we don't need to grab
dev->struct_mutex any more to prevent races when looking up the mmap
offset. Drop it and instead don't forget to use the unref_unlocked
variant (since the drm core still cares).

Aside: I stumbled over the mmap handler which directly does a
dma_mmap_attrs. But totally fails to grab a reference on the
underlying object and hence looks like it happily just leaks the ptes
since there's no guarantee the mmap isn't still around when
gem_free_object is called. Which the kerneldoc of dma_mmap_attrs
explicitly forbids.

v2: Fixup compile fail 0-day spotted.

Cc: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>