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 <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230721130741.1306563-1-tvrtko.ursulin@linux.intel.com