drm/gma500: Drop dev->struct_mutex from modeset code

It's either init code or already protected by other means. Note
that psb_gtt_pin/unpin has it's own lock, and that's really the
only piece of driver private state - all the modeset state is
protected with modeset locks already.

The only important bit is to switch all gem_obj_unref calls to the
_unlocked variant.

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/1448271183-20523-18-git-send-email-daniel.vetter@ffwll.ch