Introduce vaLockBuffer & vaUnlockBuffer APIs in libva.

Package libva and its driver supports vaLockBuffer & vaUnlockBuffer on Tizen
IVI, while the two APIs are not included in Chromium libva ATM.

https://github.com/01org/ozone-wayland/pull/260 requires the APIs to get
vaimage buffer handle so as to create EGL image.

This CL add these API in Chromium libva.