vdec: refactor sink_setcaps a bit
vdec: refactor sink_setcaps a bit

.. it got quite convoluted.

Signed-off-by: Felipe Contreras <de77a8b9cd239b204c19c52ed62e48ab47243362@nokia.com>

felipec/gst-dsp,felipec/gst-dsp,community-ssu/gst-dsp,community-ssu/gst-dsp,community-ssu/gst-dsp