drm/i915: Convert i915_gem_init_swizzling to intel_gt

Start using the newly introduced struct intel_gt to fuse together correct
logical init flow with uncore for more removal of implicit dev_priv in
mmio access.

v2:
 * Move code to i915_gem_fence_reg. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-7-tvrtko.ursulin@linux.intel.com