dmaengine: omap-dma: consolidate writes to DMA registers

There's no need to keep writing registers which don't change value in
omap_dma_start_sg().  Move this into omap_dma_start_desc() and merge
the register updates together.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>