drm/vmwgfx: unwind spaghetti code in vmw_dma_select_mode

Just use a simple if/else chain to select the DMA mode.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>