drm/i915: add immutable zpos plane properties

This adds basic immutable support for the zpos property. The zpos increases
from bottom to top: primary, sprites, cursor.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[contact@emersion.fr: adapted for latest drm-tip]
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/YSH9PasoADJJdNJCSdI4m55ankIBsCaoSgkw-NQ5dlruCAxc8J-SQwVl5n3ddSAMDLTdbdyQvkONmtbjkUU-TQk5VIu1p-aZRO1OjjuSxjY=@emersion.fr


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>