drm/i915: Protect pipe_crc->entries update

Set the pipe_crc->entries pointer while holding the relevant spinlock.
Doesn't matter too much since a spurious pipe crc interrupt would then
just update one entry but later that entry would get cleared when head
and tail are both set to 0. But being a bit more paranoid doesn't hurt.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>