sata_mv: rearrange mv_start_dma() and friends

Rearrange mv_start_dma() and friends, in preparation for adding
non-EDMA DMA modes, and non-EDMA interrupts, to the driver.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>