drm/i915/dsi: Move calling of wait_for_dsi_fifo_empty to mipi_exec_send_packet

Instead of calling wait_for_dsi_fifo_empty on all dsi ports after calling
a drm_panel_foo helper which calls VBT sequences, move it to the VBT
mipi_exec_send_packet helper, which is the one VBT instruction which
actually puts data in the fifo.

This results in a nice cleanup making it clearer what all the steps on
intel_dsi_enable / disable are and this also makes the VBT code properly
wait till a command has actually been send before executing the next
steps (typically a delay) in the VBT sequence.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/289977b5699e252fea5c211d1d1645f9e79cca79.1488273823.git.jani.nikula@intel.com