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