OMAPDSS: DISPC: Revert to older DISPC Smart Standby mechanism for OMAP5

DISPC on OMAP5 has a more optimised mechanism of asserting Mstandby to achieve
more power savings when DISPC is configured in Smart Standby mode. This
mechanism leads to underflows when multiple DISPC pipes are enabled.

There is a register field which can let us revert to the older mechanism of
asserting Mstandby. Configure this field to prevent underflows.

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