drm/i915: Simplify watermark/init_clock_gating setup

Avoid duplicating the same piece of code several times by separating
the watemark vfunc setup from the init_clock_gating vfunc setup on PCH
platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>