drm/i915: take a reference to uncore in the engine and use it

A few advantages:

- Prepares us for the planned split of display uncore from GT uncore

- Improves our engine-centric view of the world in the engine code
  and allows us to avoid jumping back to dev_priv.

- Allows us to wrap accesses to engine register in nice macros that
  automatically pick the right mmio base.

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/20190325214940.23632-10-daniele.ceraolospurio@intel.com