drm/i915: Shrink per-platform watermark configuration

Use types of more appropriate size in struct
intel_watermark_params to save 512 bytes of .rodata.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>