cell: Change code-gen for CONST_COLOR blend factor

cell: Change code-gen for CONST_COLOR blend factor

Previously the constant color blend factor was compiled into the
generated code.  This meant that the code had to be regenerated each
time the constant color was changed.  This doesn't fit with the model
used in Gallium.

As-is, the code could be better.  The constant color is loaded for
every quad processed, even if it is not used.  Also, if a lot of (1-x)
blend factors are used, 1.0 will be loaded and reloaded into registers
many times.