compositor-drm: drm_output_enable updates unused_crtcs/connectors

compositor-drm: drm_output_enable updates unused_crtcs/connectors

Let drm_output_enable() remove the CRTC and the connector from the
unused id arrays.

In the future when a list of drm_heads supersedes unused_connectors
array, the usedness of a connector will be determined by the enabled
state of the output the connector (head) is attached to. The enabled
state is turned on by drm_output_enable(). If unused_crtcs array was
still updated in drm_output_repaint(), the CRTC and connector usedness
would be tracked in different places. Logically the two belong together.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>