drm/i915: Use clamp_t() when limiting cur_delay

Make the cur_delay limiting code a bit less prone to typo errors
by using clamp_t().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>