drm: omapdrm: Don't call DISPC power handling in IRQ wait functions

The IRQ wait functions are called from the DSS enable and disable
operations only, where the DISPC is guaranteed to be enabled. There's no
need for manual DISPC power management there.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>