svga: initialize a variable to silence a gcc warning
svga: initialize a variable to silence a gcc warning

Reviewed-by: Charmaine Lee <67ce69700661ea856134ddb10419a330e5f037d7@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler