drm/i915: get/put runtime PM when we get/put a power domain

Any power domain will require the HW to be in PCI D0 state, so just do
the simple thing.

Dear maintainer: since intel_display_power_put() and
intel_display_power_get() are almost identical, git-am has failed
apply the patch on my local machine once: it added both chunks to
put(), instead of one chunk to get() and another to put(). When you
apply this patch to your tree, please check if it is correct.

v2: - Add the warning above.
v3: - Rebase.

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