drm/amd/display: Set default bits per channel

[Why]
Bump into calcReducedBlankingTiming because of mode query failed.
In this function,
timing.displayColorDepth == DISPLAY_COLOR_DEPTH_UNDEFINED.
Then req_bw == 0 because of bits_per_channel == 0.
So decide edp link settings, use default RBRx1 for special timing.

[How]
Set default bits_per_channel is 8.

Signed-off-by: Jing Zhou <Jing.Zhou@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>