i965/msaa: Work around problems with null render targets on Gen6.

On Gen6, multisampled null render targets don't seem to work
properly--they cause the GPU to hang.  So, as a workaround, we render
into a dummy color buffer.

Fortunately this situation (multisampled rendering without a color
buffer) is rare, and we don't have to waste too much memory, because
we can give the workaround buffer a very small pitch.

Fixes piglit test "EXT_framebuffer_multisample/no-color {2,4}
depth-computed *" on Gen6.

Reviewed-by: Chad Versace <[email protected]>

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