dss.c only saves 1 register (3 in OMAP3) so the extra overhead from
need_ctx_restore & co. is probably bigger than the time spent saving and
restoring those few registers every time.

So remove the code from dss.c and restore context every time dss has
been off.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>