vc4: fix release build

Mesa's DEBUG and assert's NDEBUG are not tied to each other, so we need
to explicitly compile this code out.

Fixes: 3df78928 "vc4: Drop reloc_count tracking for debug
       asserts on non-debug builds."
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 5d44e35a)