Create a new file for the DRM backend's handling of output modes, e.g.
resolution, aspect ratio, preferred mode selection, EDID parsing.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Create a new header called drm-internal.h, and move many of drm.c's
declarations and helpers to it.
This will allow us to split the DRM backend into multiple files.
Signed-off-by: Daniel Stone <daniels@collabora.com>