drm/i915: rename gen8_init_clock_gating to broadwell_init_clock_gating

Because CHV uses cherryview_init_clock_gating instead of
gen8_init_clock_gating.

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