drm/amd/display: Add aconnector condition check for dpcd read

[Why]
core_link_read_dpcd() will invoke dm_helpers_dp_read_dpcd(),
which needs to read dpcd info with the help of aconnector.
If aconnector (dc->links[i]->prev) is NULL, then dpcd status
 cannot be read.

As a result, dpcd read fails and a line of error will be
printed out in dmesg.log as:
"*ERROR* Failed to found connector for link!"

[How]
Make sure that aconnector (dc->links[i]->prev) is not NULL,
then read dpcd status.

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Joseph Gravenor <Joseph.Gravenor@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>