drm/i915: Make use of 'engine->uncore'

The engine has a direct link to the intel_uncore mmio handler, so make
use of it rather than going indirectly via &engine->i915->uncore.

v2: Update gen11_lock_sfc() to use engine->uncore as well

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405181550.7630-1-chris@chris-wilson.co.uk