drm/i915: move regs pointer inside the uncore structure

drm/i915: move regs pointer inside the uncore structure

This will allow futher simplifications in the uncore handling.

v2: move register access setup under uncore (Chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190319183543.13679-8-daniele.ceraolospurio@intel.com