drm/mediatek: hdmi: Perform NULL pointer check for mtk_hdmi_conf

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.