drm/i915: Extract general GT interrupt handlers

i915_irq.c is large. It serves as the central dispatch and handler for
all of our device interrupts. Lets break it up by pulling out the GT
interrupt handlers.

Based on a patch by Chris Wilson.

Signed-off-by: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190811210633.18417-1-chris@chris-wilson.co.uk