drm/gma500: Add driver private mutex for the fault handler

There's currently two places where the gma500 fault handler relies
upon dev->struct_mutex:
- To protect r->mappping
- To make sure vm_insert_pfn isn't called concurrently (in which case
  the 2nd thread would get an error code).

Everything else (specifically psb_gtt_pin) is already protected by
some other locks. Hence just create a new driver-private mmap_mutex
just for this function.

With this gma500 is complete dev->struct_mutex free!

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448271183-20523-21-git-send-email-daniel.vetter@ffwll.ch
diff --git a/drivers/gpu/drm/gma500/gem.c b/drivers/gpu/drm/gma500/gem.c
index f0357f5..506224b 100644
--- a/drivers/gpu/drm/gma500/gem.c
+++ b/drivers/gpu/drm/gma500/gem.c
@@ -182,7 +182,7 @@
 
 	/* Make sure we don't parallel update on a fault, nor move or remove
 	   something from beneath our feet */
-	mutex_lock(&dev->struct_mutex);
+	mutex_lock(&dev_priv->mmap_mutex);
 
 	/* For now the mmap pins the object and it stays pinned. As things
 	   stand that will do us no harm */
@@ -208,7 +208,7 @@
 	ret = vm_insert_pfn(vma, (unsigned long)vmf->virtual_address, pfn);
 
 fail:
-	mutex_unlock(&dev->struct_mutex);
+	mutex_unlock(&dev_priv->mmap_mutex);
 	switch (ret) {
 	case 0:
 	case -ERESTARTSYS: