st/mesa: fix PBO download for TEXTURE_1D_ARRAY textures

Fixes 'nir_tex_src_coord' param was provided to NIR 'txf' operation as a
vec3 for TEXTURE_1D_ARRAY target, causing an assert.
Only following targets require vec3: TEXTURE_2D_ARRAY, TEXTURE_3D,
TEXTURE_CUBE, TEXTURE_CUBE_ARRAY. The rest must use vec2.

Packing layer value into Y-coordinate the same way it was done in
'create_fs' in commit 2bf6dfac.

Fixes: a01ad311 ("st/mesa: Add NIR versions of the PBO upload/download
shaders. ")

Signed-off-by: Yevhenii Kharchenko <yevhenii.kharchenko@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <mesa/mesa!9014>