drm/layerscape: reduce excessive stack usage

The fsl-dcu driver copies a drm_mode_config object to its
stack but then only accesses a single member (dpms_property)
once. The data structure is large enough to trigger a warning
about the amount of kernel stack being used:

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_rgb.c: In function 'fsl_dcu_drm_connector_create':
drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_rgb.c:182:1: error: the frame size of 1040 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

This changes the fsl_dcu_drm_connector_create() function to
only access the drm_mode_config by reference, which is also
more efficient.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 109eee2f

 ("drm/layerscape: Add Freescale DCU DRM driver")
Signed-off-by: Stefan Agner <stefan@agner.ch>