drm/i915: Restrict gen6_reset_rps_interrupts to gen6+

Do not call gen6_reset_rps_interrupts() when we know the registers do not
exist.

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/20180812223642.24865-2-chris@chris-wilson.co.uk