vc4: unref old fence
vc4: unref old fence

Some, but not all, state trackers will explicitly unref (and set to
NULL) the previous *fence before calling pipe->flush().  So driver
should use fence_ref() which will unref the old fence if not NULL.

Signed-off-by: Rob Clark <6f78cdb10adbe0a7fbbf4098125a51f401c77a5f@freedesktop.org>
Acked-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler