vc4: Emit resets of the uniform stream at the starts of blocks.

vc4: Emit resets of the uniform stream at the starts of blocks.

If a block might be entered from multiple locations, then the uniform
stream will (probably) be at different points, and we need to make sure
that it's pointing where we expect it to be.  The kernel also enforces
that any block reading a uniform resets uniforms, to prevent reading
outside of the uniform stream by using looping.