drm/i915: Simplify enabling user-interrupts with L3-remapping

Borrow the idea from intel_lrc.c to precompute the mask of interrupts we
wish to always enable to avoid having lots of conditionals inside the
interrupt enabling.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467390209-3576-19-git-send-email-chris@chris-wilson.co.uk