i965: Listen to BRW_NEW_FRAGMENT_PROGRAM for 3DSTATE_PS_BLEND.

i965: Listen to BRW_NEW_FRAGMENT_PROGRAM for 3DSTATE_PS_BLEND.

brw_color_buffer_write_enabled depends on brw->fragment_program, which
means we have to listen to BRW_NEW_FRAGMENT_PROGRAM.

On most generations, this was only called from a function that already
subscribed.  However, on Broadwell, we failed to listen to the necessary
event in the atom that emits 3DSTATE_PS_BLEND.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>