Revert "drm/amd/display: Add helper for blanking all dp displays"

This reverts commit 1445d967.

This patch introduced a couple of dmesg warnings, this is not a valid
approach anymore. For this reason, we are reverting this patch, and we
need to revert the workaround patch.

v2: squash in unused variable removal (Alex)

Cc: Hanghong Ma <hanghong.ma@amd.com>
Cc: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>