drm/i915/scaler: Refactor max_scale computation

Refactor max scaling factor computation into a reusable
function for scalers.

--v2:
- Add missing comment. [Ankit]

Signed-off-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250120172209.188488-4-mitulkumar.ajitkumar.golani@intel.com