drm/i915: Use the i915_vma_flush_writes helper

We can use the existing helper in flush_write_domain() and save some lines
of code.

Signed-off-by: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Andi Shyti <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gem/i915_gem_domain.c

···
 	switch (obj->write_domain) {
 	case I915_GEM_DOMAIN_GTT:
 		spin_lock(&obj->vma.lock);
-		for_each_ggtt_vma(vma, obj) {
-			if (i915_vma_unset_ggtt_write(vma))
-				intel_gt_flush_ggtt_writes(vma->vm->gt);
-		}
+		for_each_ggtt_vma(vma, obj)
+			i915_vma_flush_writes(vma);
 		spin_unlock(&obj->vma.lock);
 
 		i915_gem_object_flush_frontbuffer(obj, ORIGIN_CPU);