softpipe: fix blending for luminance/intensity surfaces

softpipe: fix blending for luminance/intensity surfaces

If we're drawing to a luminance, luminance/alpha or intensity surface
we have to adjust (rebase) the fragment/quad colors before writing them
to the tile cache.  The tile cache always stores RGBA colors but if
we're caching a L/A surface (for example) we need to be sure that R=G=B
so that subsequent reads from the surface cache appear to return L/A

We previously had a special case for RGB (no alpha) surfaces.  This
change generalizes that for the other base formats.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=40408, but sRGB
formats are still failing.  That'll be addressed in a later patch.