drm/mediatek: Fix potential NULL dereference in mtk_crtc_destroy()

In the Linux kernel, the following vulnerability has been resolved: drm/mediatek: Fix potential NULL dereference in mtk_crtc_destroy() In mtk_crtc_create(), if the call to mbox_request_channel() fails then we set the "mtk_crtc->cmdq_client.chan" pointer to NULL. In that situation, we do not call cmdq_pkt_create(). During the cleanup, we need to check if the "mtk_crtc->cmdq_client.chan" is NULL first before calling cmdq_pkt_destroy(). Calling cmdq_pkt_destroy() is unnecessary if we didn't call cmdq_pkt_create() and it will result in a NULL pointer dereference.