drm/dsi: check for CONFIG_OF when defining of_mipi_dsi_device_add

drm/dsi: check for CONFIG_OF when defining of_mipi_dsi_device_add

of_mipi_dsi_device_add is used only when CONFIG_OF is enabled. It
currently works if OF support is disabled, but this will change
when we add more functionality to it.

Define the original func if CONFIG_OF is enabled. Define a dummy func
otherwise.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>