drm/exynos: drop struct_mutex from fbdev setup

Doesn't protect anything at all, and probably just here because a long
time ago dev->struct_mutex was required to allocate gem objects.

With this patch exynos is completely struct_mutex free!

Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-10-git-send-email-daniel.vetter@ffwll.ch