freedreno/a3xx: clear cached fp when switching blit prog

freedreno/a3xx: clear cached fp when switching blit prog

For gmem restore (mem2gmem), we swap blit programs, in order to have a
different frag shader for depth vs color restore.  But we weren't
actually clearing the cached fp, so it would not actually change the
frag shader as expected.

Signed-off-by: Rob Clark <robclark@freedesktop.org>