The GL frontend can do it for us now, so just use their code instead of
our own shader variants.  In the past we had to do hide the GL shader
variants in the driver to get precompiles from st, but no longer as of
!8601.

I tested with drawoverhead -test 6 (shader program change, n=30) and -test
1 (no statechanges, n=43) and saw no change in driver overhead.

Part-of: <mesa/mesa!8997>