OMAPDSS: DSI: Maintain copy of pixel format in driver data

The DSI driver currently relies on the omap_dss_device struct to receive the
desired pixel format of the panel. This makes the DSI interface driver dependent
on the omap_dss_device struct.

Make the DSI driver data maintain it's own pixel format field. The panel driver
is expected to call omapdss_dsi_set_pixel_format() to configure the pixel format
before the interface is enabled.

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