drm/i915: Use atomic page flip for intel again.

This reverts commit 527b6abe
(Revert "drm/i915: Use atomic commits for legacy page_flips")

and reapplies commit ee042aa4.
("drm/i915: Use atomic commits for legacy page_flips")

The reason for the revert was because legacy cursor updates were
forced to wait for pending page flips and rendering after they
were converted to atomic.

Commit f79f2692
(drm/i915: Add a cursor hack to allow converting legacy page flip to atomic, v3)
adds a fastpath to cursor updates, which fixes the stuttering issues.
With these changes I feel confident enough to re-enable cursor updates.

Legacy cursor update won't block in the following cases:
- Moving cursor
- Changing cursor fb

The legacy cursor update will still block in the following cases:
- Showing/hiding cursor.
- Cursor size or scaling changes.
- cursor update while cursor is invisible (could be fixed, if it turns out to be important).
- Cursor tiling changes (Not sure we support tiled cursors.)
- Last update was a modeset.

Cc: Steven Newbury <steve@snewbury.org.uk>
Cc: Rafael Ristovski <rafael.ristovski@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Rafael Ristovski <rafael.ristovski@gmail.com>
Testcase: igt/kms_cursor_legacy
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>