OMAPDSS: workaround for MFLAG + NV12 issue

It was found that having two displays enabled and having an NV12 overlay
on one of the displays will cause underflows/synclosts. Debugging this
pointed to some issue with MFLAG.

It is unclear why this issue is happening, but it looks like there is a
HW bug related to MFLAG and FIFO management. Disabling MFLAG makes this
issue go away, but then we lose the benefit of MFLAG. Also forcing MFLAG
always on makes the issue go away.

Also, using certain values for MFLAG_START, MFLAG thresholds and PRELOAD
makes the issue go away, but there was no obvious logic to which values
work and which don't.

As a workaround until more information about this is found, force MFLAG
always on to make NV12 usable.

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