drm/i915: Move intel_gt_pm_init under intel_gt_init_early

And also rename to intel_gt_pm_init_early and make it operate on gt.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-5-tvrtko.ursulin@linux.intel.com