drm/display/dp: Export fn to calculate link symbol cycles

drm/display/dp: Export fn to calculate link symbol cycles

Unify the function to calculate the link symbol cycles for both dsc and
non-dsc case and export the function so that it can be used in the
respective platform display drivers for other calculations.

v2: unify the fn for both dsc and non-dsc case (Imre)
v3: rename drm_dp_link_symbol_cycles to drm_dp_link_data_symbol_cycles
    retain slice_eoc_cycles as is (Imre)
v4: Expose only drm_dp_link_symbol_cycles() (Imre)
v6: Add slice pixels which was removed unknowingly (Vinod)

Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250424-hblank-v7-1-8b002f1506cc@intel.com