I missed that this had a micro-optimization to assume that there was
only ever one source, which is no longer valid for the bindless model
since we now have a bindless handle source. Remove the optimization to
fix assertion failures with turnip.

Fixes e.g.
dEQP-VK.glsl.texture_functions.query.texturesize.sampler2d_fixed_vertex

Part-of: <mesa/mesa!4548>