drm: omapdrm: Simplify DSS power management

Instead of sprinkling dispc_runtime_get() and dispc_runtime_put() calls
in various CRTC operations, move all power management code to the atomic
commit function.

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