fb: fix atyfb build warning

Fix build warning when neither of CONFIG_FB_ATY_GX or
CONFIG_FB_ATY_CT is enabled, since ARRAY_SIZE(aty_chips) is 0 in
that case.

drivers/video/aty/atyfb_base.c:437:11: warning: overflow in implicit constant conversion [-Woverflow]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc:	Paul Mackerras <paulus@samba.org>
Cc:	Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:	linux-fbdev@vger.kernel.org
Cc:	Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc:	Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>