drm/i915: kill pch_init_clock_gating indirection

Now that we no longer pretend to have flexibility in matching any
north display block with any pch, we can ditch this.

v2: Fix the embarassing rebase fail that Paulo Zanoni spotted.

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