drm/i915: don't suspend gt until after we disable irqs and display (v2)

When I moved the irq disable down to after display disable,
I didn't realise the gt suspend also required irqs off, so move it
down as well.

Fixes WARNs seen at suspend/resume time.

v2: moved the rps flush down as well.

Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>