drm/i915: Use a private interface for register access within GT

The GT functions for enabling register access also need to occasionally
write to and read from registers. To avoid the potential recursion as we
modify the public interface to be stricter, introduce a private register
access API for the GT functions.

v2: Rebase
v3: Rebase onto uncore
v4: Use raw interfaces consistently so that we only use the low-level
    readN functions from a single location.

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