staging: omapdrm/omap_gem_dmabuf.c: fix memory leakage

There is a memory leakage in variable sg if it goes to error.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>