CHROMIUM: drm/anx7808: fix register names and use DPCD defines from helper

Fix two register names that do not match ANX7808 register spec.
Also, use DPCD defines from drm_dp_helper.h and remove duplicates
from and7808regs.h.

BUG=none
TEST=emerge-daisy_spring chromeos-kernel-3_8

Change-Id: Id11b58cbf34a4bd6ba859d7e391dcb05fa906279
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/215368
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Merge branch 'chromeos-3.8' of https://chromium.googlesource.com/chromiumos/third_party/kernel into chromeos-3.8