Add WLSC_COMPOSITOR_IDLE state to the possible compositor internal
states, and fix the drm backend to restore the previous state instead of
forcing ACTIVE.

Normally, the compositor only uses the ACTIVE and SLEEPING states. The
IDLE state is another active state, reserved for the shell, when the
shell wants to have unlock() calls on activity, but the compositor cannot
be SLEEPING.

Use the IDLE state to fix exposing the unlock dialog while a screensaver
is animating. Without this fix, is it impossible to activate the unlock
dialog without waiting for a second idle timeout that really puts the
compositor into SLEEPING.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>