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