drm/i915: enable DP/eDP for Sandybridge/Cougarpoint

DP on Cougarpoint has new training pattern definitions, and
new transcoder DP control register is used to determine the mapping
for transcoder and DP digital output. And eDP for Sandybridge has
new voltage and pre-emphasis level definitions.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>