u_vbuf: fix src_offset alignment in u_vbuf_create_vertex_elements()

u_vbuf: fix src_offset alignment in u_vbuf_create_vertex_elements()

If the driver says PIPE_CAP_VERTEX_ELEMENT_SRC_OFFSET_4BYTE_ALIGNED_ONLY=1,
the driver should never receive a pipe_vertex_element::src_offset value
that's not a multiple of four.  But the vbuf code wasn't actually adjusting
the src_offset value when creating the vertex element state object.

We just need to align the src_offset values put in the driver_attribs[]
array.

See the piglit gl-1.5-vertex-buffer-offsets test.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>