OMAPDSS: tpo-td043 panel: fix data passing between SPI/DSS parts

This driver uses omap_dss_device that it gets from a board file through
SPI platfrom_data pointer to pass data from SPI to DSS portion of the
driver by using dev_set_drvdata(). However this trick no longer works,
as DSS core no longer uses omap_dss_device from a board file to create
the real device, so use a global pointer to accomplish this instead,
like other SPI panel drivers do.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>