drm/i915/dsi: Make intel_dsi_enable/disable directly exec VBT sequences

The drm_panel_enable/disable and drm_panel_prepare/unprepare calls are
not fine grained enough to abstract all the different steps we need to
take (and VBT sequences we need to exec) properly. So simply remove the
panel _enable/disable and prepare/unprepare callbacks and instead
export intel_dsi_exec_vbt_sequence() from intel_dsi_panel_vbt.c
and call that from intel_dsi_enable/disable().

No functional changes.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/b4ca5185d4788d92df2ed60837a24b8962a8e8ba.1488273823.git.jani.nikula@intel.com