drm/i915/display: Move squash_ctl register programming to its own function

No functional change. Introduce dg2_cdclk_squash_program and
move squash_ctl register programming bits to this.

v2: s/dg2_cdclk_squash_programming/dg2_cdclk_squash_program (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221025223042.138810-4-anusha.srivatsa@intel.com