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>