msm: vidc: change cvp buffer add sequence

[1] Due to race between cvp_register & cvp_unregister
on same cvp buffer lead to Use-After-Free issue.
[2] To address this issue, add cvp buffer into a list
inst->cvpbufs.list only after completing all required
steps on registration sequence.

Change-Id: If9f22d17e64f003528d5653f81404f3fcc6e88f5
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>