OMAPDSS: RFBI: Maintain copy of pixel size in driver data

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

Make the RFBI driver data maintain it's own pixel format field. A panel driver
is expected to call omapdss_rfbi_set_pixel_size() to configure the pixel format
before enabling the interface or calling omap_rfbi_configure().

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