[bug report] drm/mediatek: Add mbox_free_channel in mtk_drm_crtc_destroy

[Dan Carpenter]

Hello jason-jh.lin,

Commit 593b655f0523 ("drm/mediatek: Add mbox_free_channel in
mtk_drm_crtc_destroy") from Oct 28, 2021 (linux-next), leads to the
following Smatch static checker warning:

	drivers/gpu/drm/mediatek/mtk_crtc.c:132 mtk_crtc_destroy()
	warn: variable dereferenced before check 'mtk_crtc->cmdq_client.chan' (see line 130)

drivers/gpu/drm/mediatek/mtk_crtc.c
    123 static void mtk_crtc_destroy(struct drm_crtc *crtc)
    124 {
    125         struct mtk_crtc *mtk_crtc = to_mtk_crtc(crtc);
    126         int i;
    127 
    128         mtk_mutex_put(mtk_crtc->mutex);
    129 #if IS_REACHABLE(CONFIG_MTK_CMDQ)
    130         cmdq_pkt_destroy(&mtk_crtc->cmdq_client, &mtk_crtc->cmdq_handle);
                                 ^^^^^^^^^^^^^^^^^^^^^^
Dereferenced without checking.  We recently refactored these so the dereference
is detectable by static analysis.

    131 
--> 132         if (mtk_crtc->cmdq_client.chan) {
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^
Checked too late.

    133                 mbox_free_channel(mtk_crtc->cmdq_client.chan);
    134                 mtk_crtc->cmdq_client.chan = NULL;
    135         }
    136 #endif
    137 
    138         for (i = 0; i < mtk_crtc->ddp_comp_nr; i++) {
    139                 struct mtk_ddp_comp *comp;
    140 
    141                 comp = mtk_crtc->ddp_comp[i];
    142                 mtk_ddp_comp_unregister_vblank_cb(comp);
    143         }
    144 
    145         drm_crtc_cleanup(crtc);
    146 }

regards,
dan carpenter