drm/amd/display: Fix setting DP_VID_N_MUL

[Why]
Need to set VID_N_MUL for 4:2:0 cases

[How]
Move setting to enc1_stream_encoder_dp_unblank and
ensure it is also set for non-4:2:0 cases.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>