drm/i915: clarify PMINTRMSK/pm_intr_keep usage

No functional changes; just renaming a bit, tweaking a datatype,
prettifying layout, and adding comments, in particular in the
GuC setup code that touches this data.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473711577-11454-2-git-send-email-david.s.gordon@intel.com


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>