r600g/compute: Disable growing the memory pool

r600g/compute: Disable growing the memory pool

The code for growing the memory pool (which is used for storing all of
the global buffers) wasn't working.  There seem to be two separate issues
with the memory pool code.  The first was the way it was growing the pool.
When the memory pool needed more space, it would:

1. Copy the data from the memory pool's backing texture to system memory.
2. Delete the memory pool's texture
3. Create a bigger backing texture for the memory pool.
4. Copy the data from system memory into the bigger texture.

The copy operations didn't seem to be working, and I suspect that since
they were using fragment shaders to do the copy, that there might have
been a problem with the mixing of compute and 3D state.

The other issue is that the size of 1D textures is limited, and I was
having trouble getting 2D textures to work.

I think these problems will be easier to solve once more code is shared
between 3D and compute, which is why I decided to disable it for now
rather than continue searching for a fix.