drm/i915: Move intel_gt initialization to a separate file

As it will grow in a following patch make a new home for it.

v2:
 * Convert mock_gem_device as well. (Chris)

v3:
 * Rename to intel_gt_init_early and move call site to i915_drv.c. (Chris)

v4:
 * Adjust SPDX tags.
 * No need to gt/ path when including intel_gt_types.h. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-3-tvrtko.ursulin@linux.intel.com