msdkvpp: don't use NV12 as default output in normal mode

msdkvpp: don't use NV12 as default output in normal mode

If so, BGRA is the preferred output format hence BGRA will be selected
as input format by default, e.g. in the pipleline below, BGRA instead of
NV12 is selected without renegotiation, so we can avoid the NV12 issue
(see commit 3f2314a) by default.

gst-launch-1.0 videotestsrc ! msdkvpp ! glimagesink