Convert PVQ skip variable to enum

Creates the PVQ_SKIP_TYPE enum to encapsulate the different types of
skipping that can be signaled by PVQ (i.e. skip: AC, DC or both).

There is no impact on the bitstream. However, the decoder will now emit
an internal error if the decoded skip flag is out of range. The
block_skip variable is also renamed to ac_dc_coded as it stores the same
information.

Change-Id: Ib2aadaf99dc1736ea392ae5ed8948c3cdc12da9b