gallium/radeon: use unflushed fences for deferred flushes (v2)
gallium/radeon: use unflushed fences for deferred flushes (v2)

+23% Bioshock Infinite performance.

v2: - use the new fence_finish interface
    - allow deferred fences with multiple contexts
    - clear the ctx pointer after a deferred flush

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler