Strobe magic 0xB03F register to flush PCI-e GART table.

The original XGI kernel driver strobed 0xB03F each time a page was
allocated to back a GART page.  When the driver was converted to use
the DRM SG interface, this code was lost.  Returning it fixes a long
standing issue where the X-server would work fine the first time, but
acceleration commands would be ignored on the second X-server
invocation.