vc4: Add a helper for making driver-specific NIR load_uniform for GL state

vc4: Add a helper for making driver-specific NIR load_uniform for GL state

In order to move more of our lowering into NIR, we need the ability to
reference various pipeline state (like texture rectangle scaling factors
or blend colors), so we just set those up as a load_uniform with a big
offset to indicate that it's not within the shader's uniform storage and
is one of our state values.