When doing DP attach, we need to make sure the register change to take effect immediately, otherwise it may fail to catch the attach timing. BRANCH=None BUG=chrome-os-partner:28128 TEST=Display works and system boots up on Nyan and Big Original-Change-Id: I569dc435a1aa4aac0d5ecd0655d2ad87a791246d Original-Signed-off-by: Vince Hsu <vinceh@nvidia.com> Original-Reviewed-on: https://chromium-review.googlesource.com/200414 Original-Reviewed-by: Hung-Te Lin <hungte@chromium.org> Original-Reviewed-by: Jimmy Zhang <jimmzhang@nvidia.com> Original-Reviewed-by: David Hendricks <dhendrix@chromium.org> (cherry picked from commit 47b86e2893fa667bebada6a0e0b443886dd5ee02) Signed-off-by: Marc Jones <marc.jones@se-eng.com> Change-Id: Icf809b46e675bbdb8633d9a4f31d005d6644bd2a Reviewed-on: http://review.coreboot.org/7951 Tested-by: build bot (Jenkins) Reviewed-by: David Hendricks <dhendrix@chromium.org>