drm/i915: enable sdvo lvds scaling function.

Currently we implemented basic sdvo lvds function,
But except for sdvo lvds fixed mode, we can not switch
to other modes, otherwise display get black. The patch
handle three operations to enable sdvo lvds. At first
duplicate sdvo fixed mode for adjustment, then according
to fixed mode line valid all modes, at last adjust input
mode to fit our requirement.

Acked by Li Peng <peng.li@linux.intel.com>
Signed-off-by: Ma Ling <ling.ma@intel.com>
Reviewed-by: Ian Romanick <idr@freedesktop.org>
Signed-off-by: Eric Anholt <eric@anholt.net>