drm/msm/dsi: add protection against NULL dsi device

When panel probe happens after DSI probe, the DSI probe is deferred as
per current design. In the probe defer path dsi device is destroyed.
This NULL dsi device could be deferenced by the panel probe in the
mipi_dsi_attach path.

Check for NULL dsi device before accessing it.

Changes in v2:
 - Add more comments on how this NULL pointer situation will be hit

Reported-by: Jeffrey Hugo <jhugo@codeaurora.org>
Tested-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>