drm/i915/gvt: use local INTEL_GVT_OPREGION_SIZE

All of gvt uses INTEL_GVT_OPREGION_SIZE for opregion size. Follow suit
here.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8ae6e10fc0929934a14547a973312e82a4d7f7d1.1704992868.git.jani.nikula@intel.com