gst: src: Improve DMA_DRM caps selection

The translation between Pipewire parameters and Gstreamer caps is,
for compatibility reasons, ambiguous. Formats with linear modifier
are translated both in the legacy way as `format`, as well as
`drm-format`.

When finishing negotiation and setting caps, ensure that we:
1. set caps that the peer actually supports in order to prevent
   negotiation errors.
2. fixate caps to DMA_DRM if both options are supported, using the newly
   introduced helper, in order to prevent hangs.

While on it, add some small clean-ups that hopefully make the code
easier to follow, notably that `pwsrc->caps` and `pwsrc->possible_caps`
are only used during negotiation.

src/gst/gstpipewiresrc.h

@@ -55,6 +55,7 @@ struct _GstPipeWireSrc {
   gboolean autoconnect;
 
   GstCaps *caps;
+  GstCaps *possible_caps;
 
   gboolean is_video;
   GstVideoInfo video_info;