drm/i915: unify switcheroo and legacy suspend/resume handlers

By now we handle switcheroo and legacy suspend/resume the same way, so
no need to keep separate functions for them.

No functional change.

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>