Stanse found that one error path in vga_switcheroo_debugfs_write omits to
unlock vgasr_mutex.  Fix that.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NDave Airlie <airlied@redhat.com>