drm/armada: disable CRTC clock during DPMS

When the CRTC is in low power mode, it isn't running, and so there's
no point keeping the CRTC clock enabled.  Disable the CRTC clock during
DPMS.

We need to re-enable it in the mode_set callback to ensure that the
variant's compute_clock() continues to see its clock in the expected
state (enabled).

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