llvmpipe/draw: handle UBOs that are < 16 bytes.

Not sure if this is a bug in the user or not, but some CTS
tests fail due to using an 8 byte constant buffer.

Fixes: KHR-GLES31.core.layout_binding.block_layout_binding_block_VertexShader

Reviewed-by: Roland Scheidegger <sroland@vmware.com>