drm/i915/dsi: rename intel_dsi_exec_vbt_sequence to intel_dsi_vbt_exec_sequence

Use the prefix intel_dsi_vbt for all the DSI VBT functions. No
functional changes.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/0a05abca364f3bc7f9caf90c9bd3a68eef5f222f.1488810382.git.jani.nikula@intel.com