platforms/atomic-kms: Fail gracefully when bypass buffer can't be imp…

…orted for scanout.

The compositor code is expecting `framebuffer_for` to sometimes return `nullptr`; this is fine.
What's *not* fine is passing a null `fb_id` to `AtomicKmsFbHandle` and then returning a
*non*-null `Framebuffer` that's broken :)

src/platforms/atomic-kms/server/kms/display_buffer.cpp

@@ -442,6 +442,11 @@ auto mga::DmaBufDisplayAllocator::framebuffer_for(std::shared_ptr<DMABufBuffer>
 {
     auto fb_id = drm_fb_id_from_dma_buffer(drm_fd(), gbm, buffer);
 
+    if (!fb_id)
+    {
+        return {};
+    }
+
     struct AtomicKmsFbHandle : public mg::FBHandle
     {
         AtomicKmsFbHandle(std::shared_ptr<uint32_t> fb_handle, geometry::Size size) :