drm/i915: Colocate all GT access routines in the same file

Currently, the register access code is split between i915_drv.c and
intel_pm.c. It only bares a superficial resemblance to the reset of the
powermanagement code, so move it all into its own file. This is to ease
further patches to enforce serialised register access.

v2: Scan for random abuse of I915_WRITE_NOTRACE
v3: Take the opportunity to rename the GT functions as uncore. Uncore is
the term used by the hardware design (and bspec) for all functions
outside of the GPU (and CPU) cores in what is also known as the System
Agent.
v4: Rebase onto SNB rc6 fixes

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Wrestle patch into applying and inline
intel_uncore_early_sanitize (plus move the old comment to the new
function). Also keep the _santize postfix for intel_uncore_sanitize.]
[danvet: Squash in fixup spotted by Chris on irc: We need to call
intel_pm_init before intel_uncore_sanitize since the later will call
cancel_work on the delayed rps setup work the former initializes.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>