vaapiencode: fix negotiation process of output caps.

vaapiencode: fix negotiation process of output caps.

The specified caps in gst_video_encoder_set_output_state() function
arguments should not contain any resolution, pixel-aspect-ratio,
framerate, codec-data et al. Those rather should be set through the
returned GstVideoCodecState. This means that output caps creation
could be delayed until before gst_video_encoder_finish_frame() is
called.

This greatly simplifies the GstVideoEncoder::set_format() callback
by the way.