OMAP: DSS2: Clean up for dpll4_m4_ck handling

OMAP2 does not have dpll4_m4_ck source clock for dss functional clock,
but later OMAPs do. Currently we check for cpu type in multiple places
to find out if dpll4_m4_ck is available.

This patch cleans up dss.c by using the fact that dss.dpll4_m4_ck
pointer is NULL on OMAP2. This allows us to remove many of the cpu
checks.

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