drm/i915: Store backpointer to GT in uncore

We now support a per-gt uncore, yet we're not able to infer which GT
we're operating upon.  Let's store a backpointer for now.

At this point the early initialization of the gt needs to be
broken in two parts where the first is needed to assign to the gt
the i915 private data pointer and the uncore. A temporary
function has been made and the two parts are
__intel_gt_init_early() and intel_gt_init_early(). This split
will be fixed in the future with the multitile patch.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211214193346.21231-2-andi.shyti@linux.intel.com