v3dv: fix comment in texel buffer shader copy path

When using the texel buffer copy path to copy a buffer we need to sample from the buffer and for that we need a texture shader state record where we specify the base offset of the texture (the buffer). If the copy operation has a start offset we can't add that offset to the base address of the buffer because the texture state record requires the base pointer to be 64-byte aligned, so it would only work for offsets that are multiple of 64B. Instead, we pass the offset (in elements) to the shader and we use that to shift the indices into the buffer when selecting the source texel to copy.