drm/i915: Trim error mask to known engines

drm/i915: Trim error mask to known engines

For the convenience of userspace passing in an arbitrary reset mask,
remove unknown engines from the set of engines that are to be reset.
This means that we always follow a per-engine reset with a full-device
reset when userspace writes -1 into debugfs/i915_wedged.

Reported-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180316215001.12391-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>