drm/bridge: fsl-ldb: Drop DE signal polarity inversion

It's unnecessary to invert input data enable signal polarity
according to the output one. Let's drop the inversion.
Since ->atomic_check() does nothing more than the inversion,
it can be dropped entirely as well.

Without this patch, 'koe,tx26d202vm0bwa' LVDS panel connected
with i.MX8MP EVK board does not show any data on screen.

Fixes: 463db5c2 ("drm: bridge: ldb: Implement simple Freescale i.MX8MP LDB bridge")
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Marek Vasut <marex@denx.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220701065634.4027537-4-victor.liu@nxp.com