drm/i915/dsi: rename intel_dsi_pre_disable to intel_dsi_disable

The hook names reflect more the phase in the mode set sequence the hooks
are called in than what they actually do in terms of the specific
encoder. Stick to that scheme, and rename intel_dsi_pre_disable to
intel_dsi_disable. Unify the comments around this while at it. No
functional changes.

v2: Add more sense in the enable/disable hook comments (Ville)

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1488878659-10386-1-git-send-email-jani.nikula@intel.com