drm/gma500: Drop dev->struct_mutex from fbdev init/teardown code

This is init/teardown code, locking is just to appease locking checks.
And since gem create/free doesn't need this any more there's really no
reason for grabbing dev->struct_mutex.

Again important to switch obj_unref to _unlocked variants.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1448271183-20523-19-git-send-email-daniel.vetter@ffwll.ch