drm/omap: remove crtc->mgr field

In order to remove uses of 'struct omap_overlay_manager' from omapdrm,
this patch removes the crtc->mgr field.

To accomplish that, a new static array is added along the current
'omap_crtcs' static array, which is used to store the output device
connected to a crtc.

Optimally we'd use the struct omap_crtc to store this information, but
at the time when omap_crtc_dss_connect() is called, we don't yet have
the omap_crtc instances. This might possibly be fixed later, but for now
the static array does the job.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>