drm/drm_vm: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190215170546.GA30950@embeddedor

drivers/gpu/drm/drm_vm.c

@@ -584,8 +584,8 @@ static int drm_mmap_locked(struct file *filp, struct vm_area_struct *vma)
 			vma->vm_ops = &drm_vm_ops;
 			break;
 		}
-		/* fall through to _DRM_FRAME_BUFFER... */
 #endif
+		/* fall through - to _DRM_FRAME_BUFFER... */
 	case _DRM_FRAME_BUFFER:
 	case _DRM_REGISTERS:
 		offset = drm_core_get_reg_ofs(dev);
@@ -610,7 +610,7 @@ static int drm_mmap_locked(struct file *filp, struct vm_area_struct *vma)
 		    vma->vm_end - vma->vm_start, vma->vm_page_prot))
 			return -EAGAIN;
 		vma->vm_page_prot = drm_dma_prot(map->type, vma);
-	/* fall through to _DRM_SHM */
+		/* fall through - to _DRM_SHM */
 	case _DRM_SHM:
 		vma->vm_ops = &drm_vm_shm_ops;
 		vma->vm_private_data = (void *)map;