ARM: omap1: declare a dummy omap_set_dma_priority

omapfb calls directly into the omap_set_dma_priority() function in
the DMA driver. This prevents compile-testing omapfb on other
architectures. Add an inline function next to the other ones
for non-omap configurations.

Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>