drm/i915/gvt: make execlist.h self-contained

Remove unused declaration.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/e201af3bcf8e1bc389ae501ea5e3572b95e66f55.1607422863.git.jani.nikula@intel.com