drm: remove unused fields in drm_connector and document the rest

We never used initial_x/y or the force_encoder_id, so drop those fields
and proide a basic description of the others.

Really, the ELD bits belong in drm_display_info rather than directly in
the connector, but that's a separate cleanup.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>