drm/i915/lmem: make intel_region_lmem_ops static

There are no users outside of intel_region_lmem.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112172246.11933-1-jani.nikula@intel.com