agp/intel: Rename gtt_bus_addr to gtt_phys_addr

The only use of gtt_bus_addr is as an argument to ioremap(), so it is a CPU
physical address, not a bus address.  Rename it to gtt_phys_addr to reflect
this.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>