drm/i915: split general MMIO setup from per-GT uncore init

In coming patches we'll be doing the actual tile initialization between
these two uncore init phases.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211029032817.3747750-3-matthew.d.roper@intel.com