drm/amd/display: doesn't reprogram AMD OUI

[Why]
1. the corrected timing to write DPCD 300h AMD signature is before
link training.
2. and VBIOS will writes correctted AMD signature by after AGESA 1.0.0,
so driver doesn't need to write DPCD 300h again to switch config.
3. there are some OLED panel will clean backlight level to 0 once
receive DPCD 300h, so we will see flicker issue at that time.

[How]
read DPCD 300h signature before write it to avoid reprogram again, if
AMD signature was writed corrected by VBIOS, driver will not reprogram
it again.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Yu-ting Shen <Yu-ting.Shen@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>