drm/i915: Track GT interrupt handling using the master iir

Keep the master iir and use it to reduce the number of reads and writes
to the GT iir array, i.e. only the bits marked as set by the master iir
are valid inside GT iir array and will be handled during the interrupt.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180215073713.26985-1-chris@chris-wilson.co.uk