drm/i915: Use pci_iomap for remapping the MMIO registers.

Play safe and use the common routines which take care of the cachability
of the memory when setting up the iomapping for the PCI registers.
Whilst they should be cacheable for the current generations, actually
honouring what the device requires is a better long term strategy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>