drm/i915: Convert i915_gem_init_hw to intel_gt

More removal of implicit dev_priv from using old mmio accessors.

Actually the top level function remains but is split into a part which
writes to i915 and part which operates on intel_gt in order to initialize
the hardware.

GuC and engines are the only odd ones out remaining.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-15-tvrtko.ursulin@linux.intel.com