drm/i915/gt: Cancel submitted requests upon context reset

Since we process schedule-in of a context after submitting the request,
if we decide to reset the context at that time, we also have to cancel
the requets we have marked for submission.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201230220028.17089-1-chris@chris-wilson.co.uk