OMAPDSS: do not fail if dpll4_m4_ck is missing

Do not fail if dpll4_m4_ck is missing. The clock is not there on omap24xx,
so this should not be a hard error.

The patch retains the functionality before the commit 185bae10

 (OMAPDSS:
DSS: Cleanup cpu_is_xxxx checks).

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>