drm/i915/gt: Tidy repetition in declaring gen8+ interrupts

We use the same interrupt mask for each engine, so define it once in a
local and reuse.

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/20200127231540.3302516-3-chris@chris-wilson.co.uk