drm/mediatek: Remove mtk_drm_plane

Now that mtk_drm_plane just contains its base struct drm_plane, we can
just remove it and use struct drm_plane everywhere.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470279597-60453-4-git-send-email-bibby.hsieh@mediatek.com