iris: Report correct number of planes for planar images

We were only handling the modifiers case and not counting the number of
planes in actual planar images.

Fixes Piglit's ext_image_dma_buf_import-export.

Fixes: fc12fd05 ("iris: Implement pipe_screen::resource_get_param")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111509


Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
(cherry picked from commit f8887909)