drm/i915: Fix erroneous conversion to u8

adj was defined as u8. The issue is last_adj can be negative and adj is
initialized with:

  adj = dev_priv->rps.last_adj;

and we were also happily doing things like:

  if (adj < 0)

(thank static analysers!)

v2: Make new_delay an int in case we overflow the u8 in the intermediate
    computations. new_delay will get clamped at the end anyway. (Ville)

Cc: Deepak S <deepak.s@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>