drm/i915: Introduce struct intel_gt as replacement for anonymous i915->gt

We have long been slighlty annoyed by the anonymous i915->gt.

Promote it to a separate structure and give it its own header.

This is a first step towards cleaning up the separation between
i915 and gt.

v2:
 * Adjust SPDX header.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-2-tvrtko.ursulin@linux.intel.com