drm/i915: Use pci_resource functions for BARs.

This was leftover crap from kill-agp. The current code is theoretically
broken for 64b bars. (I resist removing theoretically because I am too
lazy to test).

We still need to ioremap things ourselves because we want to ioremap_wc
the PTEs.

v2: Forgot to kill the tmp variable in v1

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>