st/mesa: Record samplers for extra planes in info->textures_used.

Normally gl_nir_lower_samplers_as_deref records info->textures_used for us, but this pass runs after that, attempting to assign samplers in the same order as st_atom_texture's external_samplers_used loop so the stars align and we get the same locations.

Since we're adding textures late, we need to amend info->textures_used.

iris uses info->textures_used to set up texture bindings; this fixes Piglit's ext_image_dma_buf_import-sample-{nv12,yuv420,yvu420} there.