OMAPDSS: DPI: Maintain copy of number of data lines in driver data

The DPI driver currently relies on the omap_dss_device struct to configure the
number of data lines as specified by the panel. This makes the DPI interface
driver dependent on the omap_dss_device struct.

Make the DPI driver data maintain it's own data lines field. A panel driver
is expected to call omapdss_dpi_set_data_lines() before enabling the interface.

Signed-off-by: Archit Taneja <archit@ti.com>