Given that the buffer dcons_crom(4) exposes is used for both input

Given that the buffer dcons_crom(4) exposes is used for both input
and output, set BUS_DMA_COHERENT when creating the DMA map used for
loading the buffer. As a side-effect this solves locking issues on
sparc64 when dcons(4) calls bus_dmamap_sync(9) while in an interrupt
filter, which are executed in a critical section, and iommu(4) has
to use a sleep lock when taking advantage of the streaming buffer.

Reported and tested by:	kensmith
Approved by:		simokawa