drm/i915: fix HW lockup due to missing RPS IRQ workaround on GEN6

drm/i915: fix HW lockup due to missing RPS IRQ workaround on GEN6

In

commit dbea3cea69508e9d548ed4a6be13de35492e5d15
Author: Imre Deak <imre.deak@intel.com>
Date:   Mon Dec 15 18:59:28 2014 +0200

    drm/i915: sanitize RPS resetting during GPU reset

we disable RPS interrupts during GPU resetting, but don't apply the
necessary GEN6 HW workaround. This leads to a HW lockup during a
subsequent "looping batchbuffer" workload. This is triggered by the
testcase that submits exactly this kind of workload after a simulated
GPU reset. I'm not sure how likely the bug would have triggered
otherwise, since we would have applied the workaround anyway shortly
after the GPU reset, when enabling GT powersaving from the deferred
work.

This may also fix unrelated issues, since during driver loading /
suspending we also disable RPS interrupts and so we also had a short
window during the rest of the loading / resuming where a similar
workload could run without the workaround applied.

v2:
- separate the fix to route RPS interrupts to the CPU on GEN9 too
  to a separate patch (Daniel)

Bisected-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Testcase: igt/gem_reset_stats/ban-ctx-render
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87429
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>