[PATCH] user32: Make functions use display dc thread-safe

[吕果]

Some functions in sysparams.c invoke get_display_dc to get display dc and
  then use it without lock mechanism. it will cause condition race in multi-thread.

PS:
  I dont't kown how to make a test than can stably run into this problem, but
  call GetSystemMetrics(SM_CXSCREEN) within a tight loop in different thread
  will get zero finally.