drm/dp_mst: Store the MST PBN divider value in fixed point format

drm/dp_mst: Store the MST PBN divider value in fixed point format

On UHBR links the PBN divider is a fractional number, accordingly store
it in fixed point format. For now drm_dp_get_vc_payload_bw() always
returns a whole number and all callers will use only the integer part of
it which should preserve the current behavior. The next patch will fix
drm_dp_get_vc_payload_bw() for UHBR rates returning a fractional number
for those (also accounting for the channel coding efficiency correctly).

Cc: Lyude Paul <[email protected]>
Cc: Harry Wentland <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Wayne Lin <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Ville Syrjälä <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Acked-by: Maarten Lankhorst <[email protected]>
[Rebased changes in dm_helpers_construct_old_payload() on drm-intel-next]
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]