drm/i915: ban badly behaving contexts

Now when we have mechanism in place to track which context
was guilty of hanging the gpu, it is possible to punish
for bad behaviour.

If context has recently submitted a faulty batchbuffers guilty of
gpu hang and submits another batch which hangs gpu in quick
succession, ban it permanently. If ctx is banned, no more
batchbuffers will be queued for execution.

There is no need for global wedge machinery anymore and
it would be unwise to wedge the whole gpu if we have multiple
hanging batches queued for execution. Instead just ban
the guilty ones and carry on.

v2: Store guilty ban status bool in gpu_error instead of pointers
    that might become danling before hang is declared.

v3: Use return value for banned status instead of stashing state
    into gpu_error (Chris Wilson)

v4: - rebase on top of fixed hang stats api
    - add define for ban period
    - rename commit and improve commit msg

v5: - rely context banning instead of wedging the gpu
    - beautification and fix for ban calculation (Chris)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>