OMAP: DSS2: OMAPFB: Add struct to store per-display data

Create a new struct omapfb_display_data to contain omapfb's private
per-display data. Move the bpp override there.

This struct will be used to hold auto/manual update state of a display
in the following patches.

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