drm/i915: move modeset checks out of save/restore_modeset_reg

That way the control flow is clearer, and it prepares the stage
to extract these ums functions and hide them somewhere.

There's still tons of display stuff outside of these, but that
requires more work.

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