gralloc_gbm: add .lock_ycbcr to support video playback use case

gralloc_gbm: add .lock_ycbcr to support video playback use case

The .lock_ycbcr function hook is currently missing from gralloc driver,
and that stops video playback from working.  Add a .lock_ycbcr function
implementation to get SW video playback test start working, where
HAL_PIXEL_FORMAT_YV12 format is used/tested.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
[AmitP: Verified by playing a mp4 format file using s/w codec
        (medaswcodec) on db845c running AOSP.]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>