fb: fix atyfb unused data warnings

Fix compiler warnings of data defined but not used by using the
__maybe_unused attribute.  The date are only used with certain kconfig
settings.

drivers/video/aty/atyfb_base.c:534:13: warning: 'ram_dram' defined but not used [-Wunused-variable]
drivers/video/aty/atyfb_base.c:535:13: warning: 'ram_resv' defined but not used [-Wunused-variable]

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>