backend-drm: store EDID data

Store the EDID data as-is, so that we can tell when the EDID blob has
changed. This is not too useful yet, because all the weston_head_set_*()
API raises the device_changed flag only if the information actually
changes. However, I want to expose the libdisplay-info di_info structure
through weston_head, and those cannot be (as) easily compared.

We need to know when the EDID blob changes, so we can call
weston_head_set_device_changed() appropriately when updating di_info.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>