drm/i915/gem: Replace implicit dev_priv->uncore for stolen init

Pass around the intended intel_uncore for mmio access during stolen
setup, and avoid relying on the implicit magic I915_READ() macros.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111182143.23479-1-chris@chris-wilson.co.uk