i965/blorp: Expand clear class hierarchy to prepare for RT resolves.

i965/blorp: Expand clear class hierarchy to prepare for RT resolves.

The fragment shaders that to do color clears will be re-used to
perform so-called "render target resolves" (the resolves associated
with fast color clears).  To prepare for that, this patch expands the
class hierarchy for blorp params by adding
brw_blorp_const_color_params (which will be used for all blorp
operations where the fragment shader outputs a constant color).

Some other data structures and functions were also renamed to use
"const_color" nomenclature where appropriate.

Reviewed-by: Eric Anholt <eric@anholt.net>