drm: bring bufs code from git tree.

This checks the AGP mappings are in a valid place and also fixes the size
check in the vm..

Signed-off-by: Dave Airlie <airlied@linux.ie>
diff --git a/drivers/char/drm/drm_vm.c b/drivers/char/drm/drm_vm.c
index 7ed77e1..9e3f259 100644
--- a/drivers/char/drm/drm_vm.c
+++ b/drivers/char/drm/drm_vm.c
@@ -589,7 +589,7 @@
 		return -EPERM;
 
 	/* Check for valid size. */
-	if (map->size != vma->vm_end - vma->vm_start)
+	if (map->size < vma->vm_end - vma->vm_start)
 		return -EINVAL;
 
 	if (!capable(CAP_SYS_ADMIN) && (map->flags & _DRM_READ_ONLY)) {