OMAP: DSS2: DSI: Use platform_device pointer to get dsi data

The dsi related data structure currently creates one global instance of itself
which is accessed by dsi functions. Remove this global structure instance and
declare the struct as dsi_data. Modify dsi_init() to allocate a "dsi_data"
structure for each platform device instance. Link this data with the device's
platform_device pointer. Create the function dsi_get_dsidrv_data() which takes
the pdev and return a pointer to the device's dsi_data.

Make dsi_get_dsidev_id() return only 0 for now, this will be removed once the
name of the DSI platform device is changed to the device instance form, like
"omapdss_dsi.0" and "omapdss_dsi.1" etc.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>