The same members between QSVVPPContext and VPPContext are removed from
VPPContext, and async_depth is moved from QSVVPPParam to QSVVPPContext
so that all QSV filters using QSVVPPContext may support async depth.

In addition, we may use QSVVPPContext as base context in other QSV
filters in the future so that we may re-use functions defined in
qsvvpp.c for other QSV filters.

This commit shouldn't change the functionality of vpp_qsv / overlay_qsv.
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>