i965_drv_video: factor out and robustify vendor string.

Move out generation of the vendor string to its specific helper function,
while also making it more robust and aware of possible overflows.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
i965_drv_video: add git commit id to VA driver version string.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>