drm/i915: Add PM regs to pre/post install

drm/i915: Add PM regs to pre/post install

At the moment, these values are wiped out anyway by the rps
enable/disable. That will be changed in the next patch though.

v2: Add post install setup to address issue found by Damien in the next
patch.
replaced
WARN_ON(dev_priv->rps.pm_iir != 0);
with rps.pm_iir = 0;

With the v2 of this patch and the deferred pm enabling (which changed
since the original patches) we're now able to get PM interrupts before
we've brought up enabled rps. At this point in boot, we don't want to do
anything about it, so we simply ignore it. Since writing the original
assertion, the code has changed quite a bit, and I believe removing this
assertion is perfectly safe.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: I don't agree with the justification to drop the WARN and
added a FIXME to that effect.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>