drm/armada: fix clock counts

The DPMS handling wrt clock enables/disables was incorrect: we could end
up decrementing the clock count multiple times if we transition via
several low-power DPMS states, resulting in a kernel warning.  Fix this
by only testing to see whether we are entering or exiting the DPMS off
state.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>