QSV: choose and re-use a "preferred" implementation instead of using MFX_IMPL_AUTO_ANY.

This fixes a rare issue where hardware support was detected as available and used in encqsvInit, but MFXInit silently fell back to software in qsv_enc_init (still don't know what that's all about though).



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5759 b64f7644-9d1e-0410-96f1-a4d463321fa5