i915_irq.c is large. It serves as the central dispatch and handler for
all of our device interrupts. Pull out the GT pm interrupt handling
(leaving the central dispatch) so that we can encapsulate the logic a
little better.

Based on a patch by Chris Wilson.
Signed-off-by: NAndi Shyti <andi.shyti@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190811142801.2460-1-chris@chris-wilson.co.uk