ARM: omap1: add back omap_set_dma_priority() stub

One of my multiplatform patches went a little too far and removed
a declaration that is needed for compile-testing the omapfb
driver on non-OMAP1 platforms:

  arm-linux-gnueabi-ld: drivers/video/fbdev/omap/omapfb_main.o: in function `omapfb_do_probe':
  omapfb_main.c:(.text+0x41ec): undefined reference to `omap_set_dma_priority'

Add back the inline stub, and in turn hide the definition when
omapfb is disabled, like we do for the usb specific bits.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Fixes: 52ef8efc ("dma: omap: hide legacy interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>