drm/i915: move DP save/restore into i915_ums.c

Note that this slightly changes the order, but we only move it within
the block of registers that restore encoder state. Specifically LVDS
is now restored after DP, whereas previously it was done before.

Legacy vga is still restored afterwards, which seems to be the
important thing (if there's anything important in this restore
ordering at all).

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