drm/i915/display: Remove FRL related code from disable DP sequence for older platforms

Remove code for resetting frl related members from intel_disable_dp, as
this is not applicable for older platforms.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210309043915.1921-3-ankit.k.nautiyal@intel.com