drm/i915/gem: Merge GGTT vma flush into a single loop

We only need the one loop to find the dirty vma flush them and their
chipset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119100929.2628356-6-chris@chris-wilson.co.uk