drm/i915: Move load time init of clock gating hooks earlier

Split out the part initing the clock gating hooks and move it earlier.
Add a new NOP hook for platforms without the need to apply clockgating
or workaround settings, so that the hook can be called unconditionally.
Also add a WARN for future platforms that forget to add a hook.

The rest of the hooks in intel_init_pm() should be inited in the same
way, but atm some of the hooks are set only conditionally, so before
doing this we need to make the setup unconditional and use instead some
flags.

v2:
- add a NOP hook and WARN if no hook is set for the platform (Chris)
- use the term hook instead of callback for these functions (Jani)
v3:
- remove the GEN4() check it's already covered by earlier platform
  checks (Chris)

CC: Jani Nikula <jani.nikula@intel.com>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-6-git-send-email-imre.deak@intel.com