drm/msm/dsi: use correct enum in dsi_get_cmd_fmt

The function dsi_get_cmd_fmt returns enum dsi_cmd_dst_format,
use the correct enum value also for MIPI_DSI_FMT_RGB666/_PACKED.

This has been discovered using clang:
  drivers/gpu/drm/msm/dsi/dsi_host.c:743:35: warning: implicit conversion
        from enumeration type 'enum dsi_vid_dst_format' to different
        enumeration type 'enum dsi_cmd_dst_format' [-Wenum-conversion]
          case MIPI_DSI_FMT_RGB666:       return VID_DST_FORMAT_RGB666;
                                          ~~~~~~ ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>