i965/nir: Provide a default LOD for buffer textures

Our hardware requires an LOD for all texelFetch commands even if they are
on buffer textures.  GLSL IR gives us an LOD of 0 in that case, but the LOD
is really rather meaningless.  This commit allows other NIR producers to be
more lazy and not provide one at all.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_fs_nir.cpp

@@ -3022,6 +3022,10 @@ fs_visitor::nir_emit_texture(const fs_builder &bld, nir_tex_instr *instr)
 
    fs_reg coordinate, shadow_comparitor, lod, lod2, sample_index, mcs, tex_offset;
 
+   /* The hardware requires a LOD for buffer textures */
+   if (instr->sampler_dim == GLSL_SAMPLER_DIM_BUF)
+      lod = brw_imm_d(0);
+
    for (unsigned i = 0; i < instr->num_srcs; i++) {
       fs_reg src = get_nir_src(instr->src[i].src);
       switch (instr->src[i].src_type) {

src/mesa/drivers/dri/i965/brw_vec4_nir.cpp

@@ -1700,6 +1700,10 @@ vec4_visitor::nir_emit_texture(nir_tex_instr *instr)
                                  nir_tex_instr_dest_size(instr));
    dst_reg dest = get_nir_dest(instr->dest, instr->dest_type);
 
+   /* The hardware requires a LOD for buffer textures */
+   if (instr->sampler_dim == GLSL_SAMPLER_DIM_BUF)
+      lod = brw_imm_d(0);
+
    /* Load the texture operation sources */
    uint32_t constant_offset = 0;
    for (unsigned i = 0; i < instr->num_srcs; i++) {