drm/i915: variable renames

Name variables a bit better for copy-pasters. This got turned up as part
of review for upcoming sysfs patches.

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