drm/mipi_dsi: Create dummy DSI devices

We can have devices where the data bus is MIPI DSI, but the control bus
is something else (i2c, spi etc). A typical example is i2c controlled
encoder bridge chips.

Such devices too require passing DSI specific parameters (number of data
lanes, DSI mode flags, color format etc) to their DSI host. For a device
that isn't 'mipi_dsi_device', there is no way of passing such parameters.

Provide the option of creating a dummy DSI device. The main purpose of
this would be to attach to a DSI host by calling mipi_dsi_attach, and
pass DSI params.

Create mipi_dsi_new_dummy for creating a dummy dsi device. The driver
calling this needs to be aware of the mipi_dsi_host it wants to attach
to, and also the DSI virtual channel the DSI device intends to use.

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