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>
diff --git a/drivers/gpu/drm/omapdrm/omap_drv.c b/drivers/gpu/drm/omapdrm/omap_drv.c
index e21433c..c69a519 100644
--- a/drivers/gpu/drm/omapdrm/omap_drv.c
+++ b/drivers/gpu/drm/omapdrm/omap_drv.c
@@ -352,7 +352,7 @@
 		struct drm_connector *connector;
 		struct drm_encoder *encoder;
 		enum omap_channel channel;
-		struct omap_overlay_manager *mgr;
+		struct omap_dss_device *out;
 
 		if (!omapdss_device_is_connected(dssdev))
 			continue;
@@ -399,8 +399,10 @@
 		 * not considered.
 		 */
 
-		mgr = omapdss_find_mgr_from_display(dssdev);
-		channel = mgr->id;
+		out = omapdss_find_output_from_display(dssdev);
+		channel = out->dispc_channel;
+		omap_dss_put_device(out);
+
 		/*
 		 * if this channel hasn't already been taken by a previously
 		 * allocated crtc, we create a new crtc for it