omapdrm: use modeset_lock_all

I've left the locking in the debugfs code as-is, it's essentially just
used to keep the framebuffer object alive (which won't be necessary
any more later on). We don't need fb refcounting either, since the new
mode_config.fb_lock ensures that the framebuffers can't disappear
(once mode_config.mutex doesn't guarantee this any more later on in
the series).

The fbcon restore needs all modeset locks. The crtc callbacks seem to
only need the crtc locks, but I've quickly discussed things with Rob
Clark and he's fine with just using modeset_lock_all for those, too.
He'll look into converting things over later.

Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>