drm/i915: Move load time init of display/audio hooks earlier

All of this is SW only initialization so we can move them earlier. Move
the mutex init where the rest of the locks are inited. While at it also
convert dev to dev_priv.

v2:
- use the term hook instead of callback for these functions (Jani)

CC: Jani Nikula <jani.nikula@intel.com>
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/1458128348-15730-5-git-send-email-imre.deak@intel.com