drm/i915/dsb: Extract intel_dsb_{head,tail}()

drm/i915/dsb: Extract intel_dsb_{head,tail}()

Extract the code that calculates the DSB_HEAD/TAIL register
values into small helpers. We already have two copies of this,
and soon there will be a third.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://lore.kernel.org/r/20250523062041.166468-4-chaitanya.kumar.borah@intel.com