To count context losses, add struct i915_ctx_hang_stats for
both i915_hw_context and drm_i915_file_private.
drm_i915_file_private is used when there is no context.

v2: renamed and cleaned up the struct (Chris Wilson, Ian Romanick)
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Acked-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>