[regression] JPEG and VP8 decode fails

Since commit e962069d JPEG and VP8 decode pipelines fail on both iHD and i965 drivers:

$ gst-launch-1.0 -vf videotestsrc num-buffers=10 ! video/x-raw,format=NV12 \
  ! vaapijpegenc ! image/jpeg ! jpegparse ! filesink location=test.mjpeg

$ gst-launch-1.0 -vf filesrc location=./test.mjpeg ! jpegparse ! vaapijpegdec ! fakesink

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Got context from element 'vaapidecode_jpeg0': gst.gl.GLDisplay=context, gst.gl.GLDisplay=(GstGLDisplay)"\(GstGLDisplayX11\)\ gldisplayx11-0";
Got context from element 'vaapidecode_jpeg0': gst.vaapi.Display=context, gst.vaapi.Display=(GstVaapiDisplay)"\(GstVaapiDisplayGLX\)\ vaapidisplayglx0";
/GstPipeline:pipeline0/GstJpegParse:jpegparse0.GstPad:src: caps = image/jpeg, parsed=(boolean)true, format=(string)I420, width=(int)320, height=(int)240, framerate=(fraction)1/1
ERROR: from element /GstPipeline:pipeline0/GstJpegParse:jpegparse0: Internal data stream error.
Additional debug info:
../libs/gst/base/gstbaseparse.c(3676): gst_base_parse_loop (): /GstPipeline:pipeline0/GstJpegParse:jpegparse0:
streaming stopped, reason not-negotiated (-4)
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...

$ gst-launch-1.0 -vf videotestsrc num-buffers=10 ! video/x-raw,format=NV12 \
  ! vaapivp8enc ! video/x-vp8 ! matroskamux ! filesink location=test.webm

$ gst-launch-1.0 -vf filesrc location=./test.webm ! matroskademux ! vaapivp8dec ! fakesink

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Got context from element 'vaapidecode_vp8-0': gst.gl.GLDisplay=context, gst.gl.GLDisplay=(GstGLDisplay)"\(GstGLDisplayX11\)\ gldisplayx11-0";
Got context from element 'vaapidecode_vp8-0': gst.vaapi.Display=context, gst.vaapi.Display=(GstVaapiDisplay)"\(GstVaapiDisplayGLX\)\ vaapidisplayglx0";
WARNING: from element /GstPipeline:pipeline0/GstMatroskaDemux:matroskademux0: Delayed linking failed.
Additional debug info:
gst/parse/grammar.y(544): gst_parse_no_more_pads (): /GstPipeline:pipeline0/GstMatroskaDemux:matroskademux0:
failed delayed linking some pad of GstMatroskaDemux named matroskademux0 to some pad of GstVaapiDecode_vp8 named vaapidecode_vp8-0
ERROR: from element /GstPipeline:pipeline0/GstMatroskaDemux:matroskademux0: Internal data stream error.
Additional debug info:
../gst/matroska/matroska-demux.c(5859): gst_matroska_demux_loop (): /GstPipeline:pipeline0/GstMatroskaDemux:matroskademux0:
streaming stopped, reason not-linked (-1)
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...

cc: @vjaquez