drm/i915/gt: Reorganise gen8+ interrupt handler

We always use a deferred bottom-half (either tasklet or irq_work) for
processing the response to an interrupt which means we can recombine the
GT irq ack+handler into one. This simplicity is important in later
patches as we will need to handle and then ack multiple interrupt levels
before acking the GT and master interrupts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200127231540.3302516-2-chris@chris-wilson.co.uk