drm/i915: Squash gen lookup through multiple indirections inside GT access

The INTEL_INFO() macro extracts the dev_private pointer from the device,
so passing in the dev_private->dev is a long winded circumlocution.

v2: rebase onto uncore

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>