drm/i915: remove modeset reset from i915_reset

On gen4+ we don't reset the display unit, so resetting the complete
modeset state should not be necessary.

We can't do reset on gen3 anyway, which leaves us with gen2 reset:
According to Chris Wilson, that doesn't work so great, so he suggested
we just ignore that. If the need ever arrises, we can re-add it later
on.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>