drm/exynos: drop struct_mutex from exynos_drm_gem_get_ioctl

The only things this protects is reading ->flags and ->size, both of
which are invariant over the lifetime of an exynos gem bo. So no
locking needed at all (besides that, nothing protects the writers
anyway).

Aside: exynos_gem_obj->size is redundant with
exynos_gem_obj->base.size and probably should be removed.

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/1459330852-27668-9-git-send-email-daniel.vetter@ffwll.ch