drm/omap: Remove omap_crtc_output global array

The omap_crtc_output global array is used to look up the DSS output
device by channel. We can replace that by accessing the output device
from the pipeline if we store the pipeline pointer in the omap_crtc
structure.

The global array is also used to protect against double connection of an
output. This can't happen with the connection handling mechanism going
from DSS outputs to displays. We can thus drop that check, allowing
removal of the global array.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>