drm/i915: Pin the pages before acquiring struct_mutex for display

Since we don't need the struct_mutex to acquire the object's pages, call
i915_gem_object_pin_pages() before we bind the object into the GGTT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170726160038.29487-3-chris@chris-wilson.co.uk


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>