drm/amd/display: Remove superfluous drm_mode_config_cleanup

It's already being released by DRM core through devm

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210512142648.666476-15-andrey.grodzovsky@amd.com