drm/i915: turn on the power well before suspending

Our suspend code touches a lot of registers all over the place, so we
need to enable the power well before suspending.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Fixup compilation by stealing the header decl from the
dynamic power wells patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>