drm/i915: Add OACONTROL to the command parser register whitelist.

Mesa needs to be able to write OACONTROL in order to expose the
Observability Architecture's performance counters via OpenGL.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
[danvet: Add comment that this is just a temporary work-around and
that we need to check more things before we can allow OACONTROL writes
for real everywhere.]
[danvet 2: Squash in fixup to avoid a DRM_ERROR due to unsorted reg
list, spotted by Jani.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>