drm/i915: make runtime PM interrupt enable/disable platform independent

We need to disable the interrupts for all platforms, so make the helpers
for this platform independent and call them from them platform
independent runtime suspend/resume callbacks.

On HSW/BDW this will move interrupt disabling/re-enabling at the
beginning/end of runtime suspend/resume respectively, but I don't see
any reason why this would cause a problem there. In any case this seems
to be the correct thing to do even on those platforms.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>