iris: Fix import of multi-planar surfaces with modifiers

Multi-planar surfaces are allowed to have modifiers. Don't require
DRM_FORMAT_MOD_INVALID in order to create a surface for each plane
defined by the format.

Fixes: 246eebba ("iris: Export and import surfaces with modifiers that have aux data")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 21376cff)