drm/i915: Remove bogus FIXME from SKL wm computation

We do return an error when the watermark calculation fails, so
the FIXME claiming otherwise is outdated. Remove it.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181113172330.26069-2-ville.syrjala@linux.intel.com


Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>