compositor-drm: Pull EDID extraction into helper

Create a helper function which populates a drm_head with the information
extracted from its connector's EDID and any other properties we can
find, such as physical size and connection status.

This is currently quite small, but may become more complex in future as
we parse EDID better. It also prepares to move this function into
another file in the next commit.

Signed-off-by: Daniel Stone <daniels@collabora.com>