Adds a flag "buf_8bit_valid" to indicate if buf_8bit is synchronized
with HBD buffer, the flag is then used to avoid possible redundant
operation in down conversion the hbd buffer to buf_8bit.

Also this commit also moves the operation to prepare source frame for
coding outside of recode loop to avoid duplicate operations there.

Change-Id: I67a1eb408b75bb734bd8a16fe6ffbd58c93a230f