PIPE_CAP_TGSI_MUL_ZERO_WINS is a cap currently supported by nv50, nvc0 and
r600, which means that TGSI_PROPERTY_MUL_ZERO_WINS is supported.

When enabled, the affected shaders have the behaviour 0*anything = 0 (including
inside a mad). It is used by gallium nine when available.

All GCN cards support special legacy instructions to handle the behaviour.

Since opengl compat used to be a thing "we will never support because lack of
time" and is now implemented, I thought maybe this MUL_ZERO_WINS may get added
to the new radeonsi TODO list.