glamor: Don't initialize on softpipe
There are systems where softpipe is the default renderer, e.g. when llvmpipe is not is not avilable. Using glamor on such systems is never a good idea.
This mirrors what commit 0a9415cf did for llvmpipe.
Closes: #1417 (closed)
Signed-off-by: Ivan A. Melnikov iv@altlinux.org