intel: Only do batchbuffer debug if --enable-debug is used.
intel: Only do batchbuffer debug if --enable-debug is used.

This saves 6.6KB on the 965 driver, and appears to speed
firefox-talos-gfx up by 1-2%.  Unlike many other asserts in the
driver, when we make a mistake that would trigger one of these it
generally shows up all the time for developers, so turning it off for
release seems fine.

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