[WIN32SS:ENG/NTUSER] Use shared locks for display list lookups (#9090)

- Use shared locks for the display device and PDEV lookup paths.
  These functions only walk existing lists and take references, while list updates
  and mode-switch paths still use exclusive locking.
  This avoids serializing read-only display queries unnecessarily.

- Use shared locks for other read-only NtUser calls.

win32ss/gdi/eng/device.c

@@ -803,7 +803,7 @@ EngpFindGraphicsDevice(
            pustrDevice, iDevNum);
 
     /* Lock list */
-    EngAcquireSemaphore(ghsemGraphicsDeviceList);
+    EngAcquireSemaphoreShared(ghsemGraphicsDeviceList);
 
     if (pustrDevice && pustrDevice->Buffer)
     {

win32ss/gdi/eng/pdevobj.c

@@ -805,7 +805,7 @@ EngpGetPDEV(
     ULONG i;
 
     /* Acquire PDEV lock */
-    EngAcquireSemaphore(ghsemPDEV);
+    EngAcquireSemaphoreShared(ghsemPDEV);
 
     /* Did the caller pass a device name? */
     if (pustrDeviceName)