svga: fix blending regression

svga: fix blending regression

The earlier Mesa commit 3d06c8afb5 ("st/mesa: don't translate blend
state when it's disabled for a colorbuffer") subtly changed the
details of gallium's per-RT blend state.

In particular, when pipe_rt_blend_state[i].blend_enabled is true,
we have to get the src/dst blend terms from pipe_rt_blend_state[i],
not [0] as before.

We now have to scan the blend targets to find the first one that's
enabled (if any).  We have to use the index of that target for getting
the src/dst blend terms.  And note that we have to set identical blend
terms for all targets.

This fixes the Piglit fbo-drawbuffers2-blend test.  VMware bug 2063493.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>