drm/i915: Clear lost context-switch interrupts across reset

During a global reset, we disable the irq. As we disable the irq, the
hardware may be raising a GT interrupt that we then ignore, leaving it
pending in the GTIIR. After the reset, we then re-enable the irq,
triggering the pending interrupt. However, that interrupt was for the
stale state from before the reset, and the contents of the CSB buffer
are now invalid.

v2: Add a comment to make it clear that the double clear is purely my
paranoia.

Reported-by: "Dong, Chuanxiao" <chuanxiao.dong@intel.com>
Fixes: 821ed7df ("drm/i915: Update reset path to fix incomplete requests")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Dong, Chuanxiao" <chuanxiao.dong@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170807121919.30165-1-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20170818090509.5363-1-chris@chris-wilson.co.uk


Reviewed-by: Michel Thierry <michel.thierry@intel.com>
(cherry picked from commit 64f09f00)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>