drm/i915/display: use common function for checking scanline is moving

cpt_verify_modeset() is roughly the same as
intel_wait_for_pipe_scanline_moving(). Assume it's close enough.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bf26763caaad25f3dfcf6f8e468421f93fb99646.1673873708.git.jani.nikula@intel.com