framebuffer: don't link fb_devio into kernel image unconditionally

CONFIG_FB_DEFERRED_IO is defined as bool while CONFIG_FB is defined as
tristate. Currently fb_defio.o is linked into the kernel image even if
CONFIG_FB=m.

I fix this by updating the Makefile to link fb_defio.o into fb.o and thus
go into one place with the other core framebuffer code.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>