drm/i915: Rename gtt_bus_addr to gtt_phys_addr

We're dealing with CPU physical addresses here, which may be different from
bus addresses, so rename gtt_bus_addr to gtt_phys_addr to avoid confusion.

No functional change.

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