drm/dp: Fix comment in DP helper

Commit 4f71d0cb ("drm/dp: add a hw mutex around the transfer
functions. (v2)"), renamed the functions drm_dp_aux_register_i2c_bus()
and drm_dp_aux_unregister_i2c_bus() to drm_dp_aux_register() and
drm_dp_aux_unregister(), respectively. However, a comment referring to
the original names was not updated in the DP helper header file. Hence,
correct these names.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>