i965: Allow FS constants to be used as immediates instead of push/pull.

i965: Allow FS constants to be used as immediates instead of push/pull.

The hope is to later take advantage of the reduced constant usage to
free up regs.  This only covers the GLSL path at the moment, because
the brw_wm_emit path doesn't get the information as to whether a float
value is a constant or a uniform.