drm/i915: Reduce the time we hold struct mutex in intel_pipe_set_base()

We don't need to hold struct_mutex all through intel_pipe_set_base(),
just need to hold it while pinning/unpinning the buffers.

So reduce the struct_mutext usage in intel_pipe_set_base() just like we
did for the sprite code in:
 commit 82284b6b
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Tue Oct 1 18:02:12 2013 +0300

    drm/i915: Reduce the time we hold struct mutex in sprite update_plane code

The FBC and PSR locking is still entirely fubar. That stuff was
previouly done while holding struct_mutex, so leave it there for now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>