Display (Linux): detect all enabled or connected connectors (#1301)

DRM connectors have two properties, "enabled" and "status", standing for
"enabled by userspace" and "a display has been connected". Connector
status cannot be reliably detected sometimes, in this case status will
be set to unknown and it is still okay to enable it. Some drivers are
also able to enable disconnected connectors.

So it is both meaningful to detect connectors in two states,

- An enabled connector, where it must have been operating in some
  configuration required by userspace
- A connected connector, where driver is sure a display has been
  connected to the port, this is the obvious case.

In the original implementation, only information of connected connectors
are detected and an enabled connector without hotplug detection
capability, like a Composite video connector, will be silently ignored.
Switch to the new strategy to detect these connectors correctly.

References: https://github.com/torvalds/linux/blob/3efc57369a0ce8f76bf0804f7e673982384e4ac9/include/drm/drm_connector.h#L58-L89

Signed-off-by: Yao Zi <ziyao@disroot.org>

Author: ziyao233

src/detection/displayserver/linux/drm.c

@@ -28,13 +28,19 @@ static const char* drmParseSysfs(FFDisplayServerResult* result)
         ffStrbufAppendS(&drmDir, entry->d_name);
         uint32_t drmDirWithDnameLength = drmDir.length;
 
-        ffStrbufAppendS(&drmDir, "/status");
-        char status = 'd'; // disconnected
-        ffReadFileData(drmDir.chars, sizeof(status), &status);
-        if (status != 'c') // connected
-        {
+        char buf;
+        ffStrbufAppendS(&drmDir, "/enabled");
+        if (!ffReadFileData(drmDir.chars, sizeof(buf), &buf) || buf != 'e') {
+          /* read failed or enabled != "enabled" */
+          ffStrbufSubstrBefore(&drmDir, drmDirWithDnameLength);
+          ffStrbufAppendS(&drmDir, "/status");
+          buf = 'd';
+          ffReadFileData(drmDir.chars, sizeof(buf), &buf);
+          if (buf != 'c') {
+            /* read failed or status != "connected" */
             ffStrbufSubstrBefore(&drmDir, drmDirLength);
             continue;
+          }
         }
 
         unsigned width = 0, height = 0, physicalWidth = 0, physicalHeight = 0;