drm/i915: Add support for GPU soft reset on Ironlake.

Ironlake's graphics reset register has to be accessed via the MCHBAR,
rather than via PCI config space, which requires some refactoring.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

diff --cc drivers/gpu/drm/i915/i915_drv.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_irq.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_reg.h
Simple merge