drm/msm: Remove msm_debugfs_cleanup()

Move the contents of msm_debugfs_cleanup() to msm_drm_uninit() to free
up the drm_driver->debugfs_cleanup callback. Also remove the
mdp_kms_funcs->debugfs_cleanup callback which has no users.

Cc: robdclark@gmail.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307204924.1002-2-noralf@tronnes.org