support PAT (page table attributes) for x86.  these
are similar to mtrrs but are part of the page table.
an architecture restriction is that all entries for the
same physical memory must match.  thus i believe
this patch corrects the fact that the frame buffer
was mapped into user space as normal (write-back)
memory but mapped in the kernel as uncachable.

reference: ia32 arch. vol. 3a §10.12 "pat".