drm/i915/display: Add smem fallback allocation for dpt

Add fallback smem allocation for dpt if stolen memory
allocation failed.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220610121205.29645-1-juhapekka.heikkila@gmail.com