drm/amd/display: check dpcd read status of DP_DOWNSTREAM_PORT_0

[Why]
For special DP convert VGA dongle, core_link_read_dpcd for
DP_DOWNSTREAM_PORT_0 always return fail, then dongle_type will be set to
DISPLAY_DONGLE_NONE, which will cause desktop resolution abnormal.

[How]
If dpcd for DP_DOWNSTREAM_PORT_0 read fail, skip set dongle_type =
DISPLAY_DONGLE_NONE.

Signed-off-by: jinlong zhang <jinlong.zhang@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>