i965/nir: Provide a default LOD for buffer textures

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.