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 <[email protected]>

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Resending as I didn't get any reply in six weeks.