OMAPDSS: Fix SDI PLL locking

Commit f476ae9d (OMAPDSS: APPLY: Remove
DISPC writes to manager's lcd parameters in interface) broke the SDI
output, as it causes the SDI PLL locking to fail.

LCLK and PCLK divisors are located in shadow registers, and we normally
write them to DISPC registers when enabling the output.  However, SDI
uses pck-free as source clock for its PLL, and pck-free is affected by
the divisors. And as we need the PLL before enabling the output, we need
to write the divisors early.

It seems just writing to the DISPC register is enough, and we don't need
to care about the shadow register mechanism for pck-free. The exact
reason for this is unknown.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>