msm: vidc: Update comments in documentation of color formats

The extradata size mentioned in the documentation was misleading since
we had increased the size to 8k earlier on.

Change-Id: Ief77ced534dd0d79d9f6b272c0c6c671b8a9681d
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>