drm/i915/fbc: Extract {skl,glk}_fbc_program_cfb_stride()

Declutter gen7_fbc_activate() by sucking the override
stride programming stuff into helpers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104144520.22605-3-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>