drm/i915: Mask everything in ring HWSTAM on gen6+ in ringbuffer mode

The execlist code already masks everything in the ring HWSTAM, but
the ringbuffer code doesn't. Let's go ahead and do that. Pre-gen6
platforms setup HWSTAM during irq setup already since there's just
the one register, and it also contains bits for non-ring interrupts.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-13-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>