vc4: Skip re-emitting the shader_rec if it's unchanged.

vc4: Skip re-emitting the shader_rec if it's unchanged.

It's a bunch of work for us to emit it (and its uniforms), more work for
the kernel to validate it, and additional work for the CLE to read
it. Improves es2gears framerate by about 50%.

Signed-off-by: Eric Anholt <eric@anholt.net>