drm/mediatek: plane: Merge mtk_plane_enable into mtk_plane_atomic_update

The mtk_plane_enable is just called once by mtk_plane_atomic_update.
So, merge mtk_plane_enable into mtk_plane_atomic_update.

While we are here, also clean up the function a bit by using an fb local
variables.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470279597-60453-6-git-send-email-bibby.hsieh@mediatek.com