gst: handle blocks and size allocation for encoded format

In case of encoded video we get n_planes as 0 from the video info so
passing that as n_datas is failing during the buffer negotiation. Make
sure to use an appropriate value based on whether we have raw video or
not.

Co-authored-by: Taruntej Kanakamalla <taruntej@asymptotic.io>

src/gst/gstpipewiresrc.h

@@ -64,6 +64,7 @@ struct _GstPipeWireSrc {
   GstCaps *possible_caps;
 
   gboolean is_video;
+  gboolean is_rawvideo;
   GstVideoInfo video_info;
 #ifdef HAVE_GSTREAMER_DMA_DRM
   GstVideoInfoDmaDrm drm_info;