mesa/st: rewrite src var when lowering tex_src_plane

mesa/st: rewrite src var when lowering tex_src_plane

The assign_extra_samplers() adds the needed extra samplers but they need
to be used in the nir_tex_instr.
Otherwise the plane information is simply lost and all nir_tex_instr use the
same sampler.
Here's an example of the bug:

NIR before st_nir_lower_tex_src_plane:
	vec1 32 ssa_8 = load_const (0x00000000 /* 0.000000 */)
	vec4 32 ssa_9 = tex ssa_0 (texture_deref), ssa_0 (sampler_deref), ssa_5 (coord), ssa_8 (plane)
	vec1 32 ssa_10 = load_const (0x00000001 /* 0.000000 */)
	vec4 32 ssa_11 = tex ssa_0 (texture_deref), ssa_0 (sampler_deref), ssa_5 (coord), ssa_10 (plane)

After:

	vec4 32 ssa_9 = tex ssa_0 (texture_deref), ssa_0 (sampler_deref), ssa_5 (coord)
	vec4 32 ssa_11 = tex ssa_0 (texture_deref), ssa_0 (sampler_deref), ssa_5 (coord)

This fixes the following piglit test for radeonsi + NIR:
  - ext_image_dma_buf_import-sample_nv12
  - ext_image_dma_buf_import-sample_yuv420
  - ext_image_dma_buf_import-sample_yvu420

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>