meson: Decouple linux/dma-buf.h and libdrm checks

Currently dmabuf_dep is found when the following conditions are met:
 - linux/dma-buf.h is present;
 - libdrm is found.

This is because Linux dmabuf support requires drm_fourcc.h which is part
of libdrm.

However, dmabuf_dep is used for two purposes:
 - define HAVE_DMABUF to state dmabuf support;
 - ensure the presence of drm_fourcc.h for gdk and for the
   media-gstreamer module.

Decouple this, unconditionally check for libdrm and require it on
Linux. Then, use libdrm_dep only to state the drm_fourcc.h presence.

Given that now we unconditionally require libdrm on Linux, HAVE_DMABUF
depends only on the linux/dma-buf.h presence.

modules/media/meson.build

@@ -24,7 +24,7 @@ if gstplayer_dep.found() and gstgl_dep.found()
   cdata.set('HAVE_GSTREAMER', 1)
   media_gst_deps = [ libm, libgtk_dep, gstplayer_dep, gstgl_dep ]
 
-  if dmabuf_dep.found() and gstdrm_dep.found()
+  if libdrm_dep.found() and gstdrm_dep.found()
     cdata.set('HAVE_GSTREAMER_DRM', 1)
     media_gst_deps += [ gstdrm_dep ]
   endif