drm/mipi_dsi: refactor device creation

Create a helper function mipi_dsi_device_new which takes in struct
mipi_dsi_device_info and the mipi_dsi_host. This will be called by
of_mipi_dsi_device_add.

Instead of calling device_initialize and device_add separately, merge
it into a single device_register call. This will remove the need of
having two separate funcs mipi_dsi_device_alloc and mipi_dsi_device_add.

The reason for creating mipi_dsi_device_new is that it can also be used
as a standalone way for creating a dsi device that isn't available via
DT.

Signed-off-by: Archit Taneja <architt@codeaurora.org>