mesa: simplify and optimise vertex bindings tracking
mesa: simplify and optimise vertex bindings tracking

We only need to update it if something changes. Also
_mesa_bind_vertex_buffer() will update the mask when binding to a
NULL or default buffer so no need to do that update here.

Reviewed-by: Juan A. Suarez Romero <af79b5692270b306571bb1d8bbe0285ae3ac93d9@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler