drm/i915: Only skip requests once a context is banned

drm/i915: Only skip requests once a context is banned

If we skip before banning, we have an inconsistent interface between
execbuf still queueing valid request but those requests already queued
being cancelled. If we only cancel the pending requests once we stop
accepting new requests, the user interface is more consistent.

Reported-by: Tvrtko Ursulin <[email protected]>
Fixes: 821ed7df6e2a ("drm/i915: Update reset path to fix incomplete requests")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: <[email protected]> # v4.9+
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]