drm/i915: debugfs interface to manually reset the GPU

Create a /debug/dri/%d/i915_wedged file to display the current wedged
status, and to enable setting that value. On an i965, this will also
trigger a GPU reset.

Useful in order to attempt to recover from some error conditions that
are not currently caught by the automatic hang detection code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>