OMAPDSS: Remove old fifomerge hacks

Once in a time omapdss had basic support for fifomerge. Fifomerge was
removed as the implementation didn't work properly, and a proper
implementation is a complex problem.

However, some unused fifo-merge related code was left behind. This patch
removes those.

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