drm/imx: atomic phase 3 step 2: Legacy callback fixups

Now that we can use atomic configurations, all the legacy callbacks
of CRTCs, encoders and connectors can be switched to the atomic version.
For the imx-ldb driver, there is a clock parent setting mismatch bewteen
->enable and ->disable after the switch, so a fixup is added.  For the
imx-tve driver, since the encoder's callback ->dpms is replaced by
->disable, we need to move the setting for the IPU_CLK_EN bit(in register
TVE_COM_CONF_REG) from ->enable/->disable to ->mode_set, otherwise, the
relevant CRTC cannot be disabled correctly with a warning on DC stop timeout.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>