drm: reorganise minor number handling using backported modesetting code.

rips out the head crap and replaces it with an idr and drm_minor structure

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/char/drm/drm_agpsupport.c b/drivers/char/drm/drm_agpsupport.c
index 9468c78..aefa5ac 100644
--- a/drivers/char/drm/drm_agpsupport.c
+++ b/drivers/char/drm/drm_agpsupport.c
@@ -122,7 +122,7 @@
 int drm_agp_acquire_ioctl(struct drm_device *dev, void *data,
 			  struct drm_file *file_priv)
 {
-	return drm_agp_acquire((struct drm_device *) file_priv->head->dev);
+	return drm_agp_acquire((struct drm_device *) file_priv->minor->dev);
 }
 
 /**