drm/i915: switch uncore mmio funcs to use intel_uncore

The full read/write ops can now work on the intel_uncore struct.
Introduce intel_uncore_read/write functions working on intel_uncore
and switch the I915_READ/WRITE macro to internally call those.

v2: no change
v3: add intel_uncore_read/write functions (Chris), update commit msg

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>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325214940.23632-6-daniele.ceraolospurio@intel.com