drm/imx: imx-ldb: Check for null pointer after calling kmemdup

As the possible failure of the allocation, kmemdup() may return NULL
pointer.
Therefore, it should be better to check the return value of kmemdup()
and return error if fails.

Fixes: dc80d703

 ("drm/imx-ldb: Add support to drm-bridge")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20220105074729.2363657-1-jiasheng@iscas.ac.cn