drm/i915/gem: Cancel contexts when hangchecking is disabled

Normally, we rely on our hangcheck to prevent persistent batches from
hogging the GPU. However, if the user disables hangcheck, this mechanism
breaks down. Despite our insistence that this is unsafe, the users are
equally insistent that they want to use endless batches and will disable
the hangcheck mechanism. We are looking at replacing hangcheck, in the
next patch, with a softer mechanism, that sends a pulse down the engine
to check if it is well. We can use the same preemptive pulse to flush an
active context off the GPU upon context close, preventing resources
being lost and unkillable requests remaining on the GPU after process
termination.

Testcase: igt/gem_ctx_exec/basic-nohangcheck
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191023133108.21401-4-chris@chris-wilson.co.uk