fbdev: sis: enforce selection of at least one backend

The sis framebuffer driver complains with a compile-time warning
if neither the FB_SIS_300 nor FB_SIS_315 symbols are selected:

drivers/video/fbdev/sis/sis_main.c:61:2: warning: #warning Neither CONFIG_FB_SIS_300 nor CONFIG_FB_SIS_315 is se

This is reasonable because it doesn't work in that case, but it's
also annoying for randconfig builds and is one of the most common
warnings I'm seeing on ARM now.

This changes the Kconfig logic to prevent the silly configuration,
by always selecting the FB_SIS_300 variant if the other one is
not set.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>