drm/i915: Replace the .modeset_commit_cdclk() hook with a more direct .set_cdclk() hook

With the cdclk state, all the .modeset_commit_cdclk() hooks are
now pointless wrappers. Let's replace them with just a .set_cdclk()
function pointer. However let's wrap that in a small helper that
does the state comparison and prints a unified debug message across
all platforms. We didn't even have the debug print on all platforms
previously. This reduces the clutter in intel_atomic_commit_tail() a
little bit.

v2: Wrap .set_cdclk() in intel_set_cdclk()
v3: Add kernel-docs
v4: Deal with IS_GEN9_BC()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126195201.32638-1-ville.syrjala@linux.intel.com