gallium: add fragment shader property for color writes to all buffers. (v2)

gallium: add fragment shader property for color writes to all buffers. (v2)

For GL fragColor semantics we need to tell the pipe drivers that the fragment
shader color result is to be replicated to all bound color buffers, this
adds the basic TGSI + documentation.

v2: fix missing comma pointed out by Tilman on mesa-dev.

Signed-off-by: Dave Airlie <airlied@redhat.com>