drm/ingenic: Register devm action to cleanup encoders

Since the encoders have been devm-allocated, they will be freed way
before drm_mode_config_cleanup() is called. To avoid use-after-free
conditions, we then must ensure that drm_encoder_cleanup() is called
before the encoders are freed.

v2: Use the new __drmm_simple_encoder_alloc() function

v3: Use the new drmm_plain_simple_encoder_alloc() macro

v4: Use drmm_plain_encoder_alloc() macro

Fixes: c369cb27

 ("drm/ingenic: Support multiple panels/bridges")
Cc: <stable@vger.kernel.org> # 5.8+
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210327115742.18986-4-paul@crapouillou.net