drm/i915/dp: Separate out helper for compute fec_enable

Make a separate function for setting fec_enable in crtc_state.

v2: Rename helper to align with encoder->compute_config() callback
and other minor fixes. (Jani)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241217093244.3938132-4-ankit.k.nautiyal@intel.com