vc4: Don't try to do stores to buffers that aren't bound.

vc4: Don't try to do stores to buffers that aren't bound.

The code was kind of mixed up what buffers were getting stored in the case
that a resolve bit was unset (which are set based on the GL state at draw
time) and the buffer wasn't actually bound.  In particular, depth-only
rendering would store the color buffer contents, which happen to be
pointing at the depth buffer.

Thanks to clearing out the resolve bits for things we really can't
resolve, now I can drop the safety checks for buffer presence around the
actual stores.

Fixes 42 piglit tests.