An output surface is returned but without sync point when when
MFXVideoDECODE_DecodeFrameAsync () returns MFX_ERR_MORE_DATA, this
surface should be released too, otherwise the surface is occupied
and it is easy to exhaust all pre-allocated mfx surfaces.

Example pipeline (input_vp8.webm contains lots of frame with show_frame
set to 0):

gst-launch-1.0 filesrc location=input_vp8.webm ! matroskademux !
msdkvp8dec ! msdkvpp ! fakesink

0:00:05.995959693 19866 0x563f30f14590 ERROR                default
gstmsdkvideomemory.c:77:gst_msdk_video_allocator_get_surface: failed to
get surface available
ERROR: from element
/GstPipeline:pipeline0/GstMatroskaDemux:matroskademux0: Internal data
stream error.