flush_write_domain() is only used within the GEM domain management code,
so move it to i915_gem_domain.c and drop the export.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMatthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119144912.12653-5-chris@chris-wilson.co.uk