drm/armada: don't grab dev->struct_mutex for in mmap offset ioctl

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).

v2: Split out the leak fix in dump_map_offset into a separate patch as
requested by Russell. Also align labels the same way as before to
stick with local coding style.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>