CVE-2021-47108

In the Linux kernel, the following vulnerability has been resolved:
drm/mediatek: hdmi: Perform NULL pointer check for mtk_hdmi_conf In commit
41ca9caaae0b (“drm/mediatek: hdmi: Add check for CEA modes only”) a check
for CEA modes was added to function mtk_hdmi_bridge_mode_valid() in order
to address possible issues on MT8167; moreover, with commit c91026a938c2
(“drm/mediatek: hdmi: Add optional limit on maximal HDMI mode clock”)
another similar check was introduced. Unfortunately though, at the time of
writing, MT8173 does not provide any mtk_hdmi_conf structure and this is
crashing the kernel with NULL pointer upon entering
mtk_hdmi_bridge_mode_valid(), which happens as soon as a HDMI cable gets
plugged in. To fix this regression, add a NULL pointer check for hdmi->conf
in the said function, restoring HDMI functionality and avoiding NULL
pointer kernel panics.