drm/i915/gt: Tidy up rps irq handler to use intel_gt

Since the rps is tied to its intel_gt, use that backpointer to find the
right engine rather than delving into i915.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191027175505.25470-1-chris@chris-wilson.co.uk