OMAP: DSS2: Remove usage of struct dispc_reg

struct dispc_reg was originally used while migrating from old omapfb to
catch cases where the arguments to dispc_read_reg/dispc_write_reg were in
wrong order, since old omapfb had the arguments in reverse order.

Remove this struct and use u16 instead

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