the DRM_{READ,WRITE}$NUMBER() macros currently use

the DRM_{READ,WRITE}$NUMBER() macros currently use
bus_space_{read,write}(). A bug in the code this was based on means that
this is also used for scatter gather maps, which are bus_dma memory, not
pci memory.  This obviously fails. Pull them out into a function and
Special case scatter gather to use the mapped virtual address to write
instead. Makes writeback test pass on pci and pci express radeon cards.

Tested by, among others ckuethe@ and sthen@.