drm/i915/gem: Track ggtt writes from userspace on the bound vma

When userspace writes into the GTT itself, it is supposed to call
set-domain to let the kernel keep track and so manage the CPU/GPU
caches. As we track writes on the individual i915_vma, we should also be
sure to mark them as dirty.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119112515.2766748-1-chris@chris-wilson.co.uk