drm/i915: Add a cursor hack to allow converting legacy page flip to atomic, v3.

Do something similar to vc4, only allow updating the cursor state
in-place through a fastpath when the watermarks are unaffected. This
will allow cursor movement to be smooth, but changing cursor size or
showing/hiding cursor will still fall back so watermarks can be updated.

Only moving and changing fb is allowed.

Changes since v1:
- Set page flip to always_unused for trybot.
- Copy fence correctly, ignore plane_state->state, should be NULL.
- Check crtc_state for !active and modeset, go to slowpath if the case.
Changes since v2:
- Make error handling work correctly. (Matthew Auld)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/a8e4cb00-5171-14e5-bbe3-dadb654ff296@linux.intel.com