drm/amd/display: Add timing pixel encoding for mst mode validation

[Why] Mode pbn is not calculated correctly because timing pixel encoding is
not checked within convert_dc_color_depth_into_bpc.

[How] Get mode kbps from dc_bandwidth_in_kbps_from_timing, then calculate
pbn by kbps_to_peak_pbn.

Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>