Revert "drm/amd/display: disable dcn20 abm feature for bring up"

This reverts commit 96cb7cf1.

This change was used for DCN2 bringup and is no longer desired.
In fact it breaks backlight on DCN2 systems.

Cc: Alexander Monakov <amonakov@ispras.ru>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Anthony Koo <Anthony.Koo@amd.com>
Cc: Michael Chiu <Michael.Chiu@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reported-and-tested-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>