drm: Pass the drm_dp_aux->hw_mutex to i2c for its locking

Rather than have both drm_dp_aux lock within its transfer, and i2c to
lock around the transfer, use the same lock by filling in the locking
callbacks that i2c wants to use. We require our own hw_mutex as we
bypass i2c_transfer for drm_dp_dpcd_access().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466152398-20157-2-git-send-email-chris@chris-wilson.co.uk