fbdev: FB_OPENCORES should depend on HAS_DMA

If NO_DMA=y:

    drivers/built-in.o: In function `ocfb_remove':
    ocfb.c:(.text+0x27fee): undefined reference to `dma_free_coherent'
    drivers/built-in.o: In function `ocfb_probe':
    ocfb.c:(.text+0x28418): undefined reference to `dma_alloc_coherent'
    ocfb.c:(.text+0x284d2): undefined reference to `dma_free_coherent'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>