drm/i915/region: make intel_region_map static

There are no users outside of intel_memory_region.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/20210112170429.27619-1-jani.nikula@intel.com