drm/i915/gem: Make i915_gem_object_flush_write_domain() static

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: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119144912.12653-5-chris@chris-wilson.co.uk