drm: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/11[e-mail]

drivers/gpu/drm/drm_bufs.c

···
 			if ((entry->map->offset & 0xffffffff) ==
 			    (map->offset & 0xffffffff))
 				return entry;
+			break;
 		default: /* Make gcc happy */
 			;
 		}