drm/exynos: Drop dev->struct_mutex from mmap offset function

Simply forgotten about this when I was doing my general cleansing of
simple gem mmap offset functions. There's nothing but core functions
called here, and they all have their own protection already.

Aside: DRM_ERROR for userspace controlled input isn't great, but
that's for another patch.

v2: Use _unlocked unreference (Daniel Stone).

Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-7-git-send-email-daniel.vetter@ffwll.ch