drm/omap: remove use of omapdss_find_mgr_from_display()

In order to remove uses of 'struct omap_overlay_manager' we need to get
rid of using omapdss_find_mgr_from_display() when initializing omapdrm.

Instead of using omapdss_find_mgr_from_display() and mgr->id to find the
dispc channel used for the given display, we can instead use
omapdss_find_output_from_display(), and get the output->dispc_channel
from there.

We can also remove omapdss_find_mgr_from_display() as it's no longer
used.

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