drm/i915: assert we're not runtime suspended when accessing registers

I could swear this was already happening in the current code...

Also, put the reads and writes in a generic place, so we don't forget
it again when we add runtime PM support to new platforms.

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