mesa: enable AMD_blend_minmax_factor

... where it can be.
nouveau: allow AMD_blend_minmax_factor

Sound strange if nvidia would had chosen a different
implementation of min/max blending (where even
intel i830 allow factors with min/max blending).
Indeed, the driver doesn't set src and dst factors to ONE
when GL_MIN or GL_MAX are set as blend equation. Maybe
Francisco Jerez (going to ask him) simply forgot about
that and nv10-20 support it or nVIDIA had clearly opted
to another form of hardware implementation.
Anyway. Can't test this right now (lacks nv10 hw).