drm/via: Add new condition to via_dma_cleanup()

Pointer dev_priv->mmio, which was checked for NULL at via_do_init_map(),
is passed to via_do_cleanup_map() and is dereferenced there without check.

The patch adds the condition in via_dma_cleanup() which prevents potential NULL
pointer dereference.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 22f579c6 ("drm: Add via unichrome support")
Signed-off-by: Alisa Khabibrakhmanova <khabibrakhmanova@ispras.ru>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220729090643.240778-1-khabibrakhmanova@ispras.ru