drm/imx: Kconfig: Remove duplicated 'select DRM_KMS_HELPER' line

A duplicated line 'select DRM_KMS_HELPER' was introduced in Kconfig file
by commit 09717af7 ("drm: Remove CONFIG_DRM_KMS_CMA_HELPER option"),
so remove it.

Fixes: 09717af7

 ("drm: Remove CONFIG_DRM_KMS_CMA_HELPER option")
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221009023527.3669647-1-victor.liu@nxp.com