va: decoder api refactor
va: Refactor _set_format() to _set_frame_size().
Renamed gst_va_decoder_set_format() to
gst_va_decoder_set_frame_size_with_surfaces() which resembles better
the passed parameters. Internally it creates the vaContext.
Added gst_va_decoder_set_frame_size() which is an alias of
gst_va_decoder_set_frame_size_with_surfaces() without surfaces. This
is the function which replaces gst_va_decoder_set_format() where
used.
va: Refactor _change_resolution() to _update_frame_size().
Rename gst_va_decoder_change_resolution() to
gst_va_decoder_update_frame_size() which resembles
gst_va_decoder_set_frame_size().
Also added a comment to clarify the function use and makes more
specific the error message.
va: Refactor _format_changed() to _config_is_equal().
Change gst_va_decoder_format_changed() to
gst_va_decoder_config_is_equal(), which is more similar with other
GStreamer API.
The function call is replaced but it has to be negated because the
return value is the opposite.
va: decoder: Group decoder methods.
Move up gst_va_decoder_get_config() to group decoders function in the
same file area.
va: vp9dec: Minor cleanups.
Added a comment with a future to-do, enhanced another comment and
fixed a typo in an error log message.