ARM: OMAP: Split fb.c to remove last remaining cpu_is_omap usage

Looks like we missed plat-omap/fb.c for cpu_is_omap usage
mach-omap2. This is the last user of cpu_is_omap, so let's
quickly fix it up so we can finally remove plat/cpu.h for
omap2lus.

We want to limit cpu_is_omap macro usage to mach-omap2 only so
we can make plat/cpu.h private. After this we can finally drop
plat/cpu.h for omap2+.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>