This also avoids having 'bit_depth'/'bd' members whose presence
is decided by the preprocessor.

The goal is to simplify statements accessing the bitdepth value.
(instead of forcing them to also rely on the preprocessor to query
the member presence through #if CONFIG_HIGHBIDEPTH).

Change-Id: I68b822bbaa2b1c67bde81cb4f3f8b9988b798b0d