cgem: improve usage of busdma(9) KPI

BUS_DMA_NOCACHE should only be used when one needs to guarantee the
created mapping has uncached memory attributes, usually as a result
of buggy hardware. Normal use cases should pass BUS_DMA_COHERENT, to
create an appropriate mapping based on the flags passed to
bus_dma_tag_create().

This should have no functional change, since the DMA tags in this driver
are created without the BUS_DMA_COHERENT flag.

Reported by: mmel
Reviewed by: mmel, Thomas Skibo <thomas-bsd@skibo.net>
MFC after: 3 days