drm/i915: Flush GT idle status upon reset

drm/i915: Flush GT idle status upon reset

Upon resetting the GPU, we force the engines to be idle by clearing
their request lists. However, I neglected to clear the GT active status
and so the next request following the reset was not marking the device
as busy again. (We had to wait until any outstanding retire worker
finally ran and cleared the active status.)

Fixes: 9b6fd824cce5 ("drm/i915: Only start retire worker when idle")
Testcase: igt/pm_rps/reset
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468397438-21226-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
(cherry picked from commit b913b33c43db849778f044d4b9e74b167898a9bc)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>