drm/i915: disable/re-enable PCI device around S4 freeze/thaw

We already disable everything during S4 freeze, except the PCI device
itself. There is no reason why we couldn't disable that too and doing
so allows us to unify these handlers in the next patch with the
corresponding S3 suspend/resume handlers.

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>