drm: Fix locking cargo-cult in encoder/plane init/cleanup

Encoders&planes can't be hotplugged, we dont need locking for this
since it's all single-threaded driver setup/teardown code. CRTCs
already don't grab locks.

While at it I noticed that plane's are missing the
drm_modeset_lock_fini() call, so add it.

Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161129094538.9650-1-daniel.vetter@ffwll.ch