Re-enable old FFmpeg's VDPAU API for non-nvidia drivers

This is a partial reverse of a5b996c06dd73b378b39bbc772183c4abf53e4d7.
The new FFmpeg hwaccel API causes issues with non-nvidia drivers, causing either crashes (nouveau drivers) or playback issues (AMD).
So only use hwaccel with NVIDIA native drivers.
As of today (2014-05-20) The Ondine_clip.mpg doesn't play with any non nvidia drivers anyway (ref #11377)

This is a very direty solution, I don't like it but I couldn't find a universal solution working on all platforms at once.
Hopefully, hwaccel in FFmpeg 2.x works better
Don't attempt to test for GPU decoding methods once one has ben found

It is now possible with video cards such as AMD to have support for both VAAPI and VDPAU; which would cause myth to attempt to use both at once.
There's also no need to test for GPU methods if hardware decoding is disabled.

Simplify code while at it.

(cherry picked from commit 5002c1ca51ec8ed9367e54a517e1ad4f378fff78)