DRM: Add support for HDMI content type

Some monitors expose a selector for the kind of content that will get
displayed, allowing them to optimise their settings for this particular
content type.

I got access to such a monitor, sadly even setting it to game mode
didn’t lower its atrocious latency, but drm_info[1] reports it to be set
correctly so hopefully it’ll work better with other monitors.

[1] https://github.com/ascent12/drm_info



Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>