r600g: Drop references to destroyed blend state
r600g: Drop references to destroyed blend state

Fixes use-after-free when the currently bound blend state is destroyed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85267
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84140

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>

Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler