drm/imx: imx-ldb: enable DI clock in encoder_mode_set

Commit eb10d635 ("imx-drm: encoder prepare/mode_set must use adjusted mode")
broke the first LVDS modeset by using crtc->hwmode before crtc mode_set is
called. In fact, encoder prepare is not supposed to prepare the display clock
at all. Rather encoder mode_set should be used to set the DI clock rate, before
it is enabled by crtc commit.

Reported-by: Liu Ying <Ying.Liu@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>