vtdec: make vtdec_hw fallback to software on renegotiation

vtdec: make vtdec_hw fallback to software on renegotiation

When renegotiating mid stream - for example with variable bitrate
streams - and therefore destroying and recreating VTSessions, the
hw decoder might become temporarily unavailable.

To deal with this and avoid erroring out on bitrate changes,
vtdec_hw now falls back to using the software decoder if the hw
one was available at some point but isn't anymore. At
renegotiation/bitrate change time, it will still retry to open
the hardware one.