Revert "drm/i915: Remove i915_gem_object_get_dirty_page()"

These commits caused a regression on Lenovo t520 sandybridge
machine belonging to reporter. We are reverting them for 5.10
for other reasons, so just do it for 5.9 as well.

This reverts commit 763fedd6

.

Reported-by: Harald Arnesen <harald@skogtun.org>
Signed-off-by: Dave Airlie <airied@redhat.com>