drm: Avoid drm_global_mutex for simple inc/dec of dev->open_count

Since drm_global_mutex is a true global mutex across devices, we don't
want to acquire it unless absolutely necessary. For maintaining the
device local open_count, we can use atomic operations on the counter
itself, except when making the transition to/from 0. Here, we tackle the
easy portion of delaying acquiring the drm_global_mutex for the final
release by using atomic_dec_and_mutex_lock(), leaving the global
serialisation across the device opens.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Reviewed-by: Thomas Hellström <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124130107.125404-1-chris@chris-wilson.co.uk

include/drm/drm_device.h

@@ -144,7 +144,7 @@ struct drm_device {
 	 * Usage counter for outstanding files open,
 	 * protected by drm_global_mutex
 	 */
-	int open_count;
+	atomic_t open_count;
 
 	/** @filelist_mutex: Protects @filelist. */
 	struct mutex filelist_mutex;