Add mechanism to set/unset the DISPC fifo-merge:

Add new fields to dss_data, fifo_merge and fifo_merge_dirty. These are
similar to the other info/dirty flags in ovl_priv_data and ovl_mgr_data,
but fifo merge is a common attribute to all managers and thus outside
the ovl_mgr_data.

The fifo-merge field is used in the dss_write_regs_common, which handles
writing the register.

dss_apply_fifo_merge() can be used to set/unset the fifo merge field in
the dss_data.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>