OMAPDSS: FEAT: Add FIFO_MERGE feature

Add feature flag for fifo merge. OMAP2 doesn't contain fifo merge, later
OMAPs do.

dispc_enable_fifomerge() checks for the flag when called, and gives a
WARN if fifo merge is being enabled when it is not supported.

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