omapfb: dispc: disable iface clocks along with func clocks

Leaving interface clocks enabled causes dss pwrdm to stay in active state
when mpu is in active state.  This fix puts dss to sleep state when it is
not needed.

Earlier version broke framebuffer on 24xx.  This is fixed by enabling
clocks before trying to access DISPC_IRQSTATUS register.

Signed-off-by: Jouni Hogander <jouni.hogander@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Imre Deak <imre.deak@nokia.com>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>