vc4: Skip storing the Z/S contents when it's invalidated.
vc4: Skip storing the Z/S contents when it's invalidated.

Improves framerate of 5 seconds of es2gears by 1.57473% +/- 0.669409%
(n=67).

Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler