st: Lower uniforms in st in the !PIPE_CAP_PACKED_UNIFORMS case as well.

PIPE_CAP_PACKED_UNIFORMS conflates several things: Lowering uniforms i/o at the st level instead of the backend, packing uniforms with no padding at all, and lowering to UBOs.

Requiring backends to lower uniforms i/o for !PIPE_CAP_PACKED_UNIFORMS leads to the driver needing to either link against the type size function in mesa/st, or duplicating it in the backend. Given that all backends want this lower-io as far as I can tell, just move it to mesa/st to resolve the link issue and avoid the driver author needing to understand st's uniforms layout.