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/psb_drv.h b/drivers/gpu/drm/gma500/psb_drv.h
index e21726e..3bd2c72 100644
--- a/drivers/gpu/drm/gma500/psb_drv.h
+++ b/drivers/gpu/drm/gma500/psb_drv.h
@@ -465,6 +465,8 @@
 	struct mutex gtt_mutex;
 	struct resource *gtt_mem;	/* Our PCI resource */
 
+	struct mutex mmap_mutex;
+
 	struct psb_mmu_driver *mmu;
 	struct psb_mmu_pd *pf_pd;