ARM: pxa168fb: clear enable bit when not active

While fb isn't active, we should clear CFG_GRA_ENA bit. The existing code
can't clear this bit.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>