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.
10 jobs for !897 with iris-yuv in 8 minutes and 52 seconds (queued for 3 seconds)
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Status Job ID Name Coverage
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passed #301310
debian

00:00:24

 
  Build+Test
passed #301312
meson-clang

00:08:28

passed #301316
meson-clover

00:08:13

passed #301315
meson-main

00:04:08

passed #301311
meson-swr-glvnd

00:06:17

passed #301317
meson-vulkan

00:03:40

passed #301319
scons-llvm

00:04:09

passed #301318
scons-nollvm

00:03:32

passed #301313
scons-swr

00:02:07

passed #301314
scons-win64

00:07:04