drm/i915: remove unused function declaration

This function is not part of the driver anymore.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 90f4fcd5 ("drm/i915: Remove forced stop ring on suspend/unload")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170804140348.24971-1-lionel.g.landwerlin@intel.com