freedreno/a3xx: assert() -> debug_assert()

We hit this assert with some piglit tests.  Which appears to be a bug
outside of freedreno.  Previously we were relying on assert() being
redefined to debug_assert() so that we didn't crash in release builds.
Somehow that stopped working.  So just use debug_assert() directly.

Signed-off-by: Rob Clark <robclark@freedesktop.org>