i915/drm/gt: Move the gt defines in the gt directory

Create a new intel_gt_defines.h inside the gt/ directory as a
placeholder for all the generic GT based defines.

As of now place only I915_MAX_GT.

Co-developed-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230801141955.383305-3-andi.shyti@linux.intel.com