drm/i915: check if IIR is still zero at postinstall on Gen5+

It should already be masked and disabled and zeroed at the preinstall
and uninstall stages. Also, the current code just writes to IIR once,
and this is not a guarantee that it will be cleared, so it's wrong
anyway.

The whole reason for the paranoia is that we're going to start calling
the IRQ preinstall/postinstall/uninstall from the runtime PM
callbacks, so we need to make sure everything is behaving as expected.

v2: - Change the original DRM_ERROR to WARN and clear IIR in case it's
      not zero (Ben).
    - Improve commit message (Daniel).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>