drm/mxsfb: Switch to drmm_mode_config_init

Switch from deprecated unmanaged drm_mode_config_init() to
managed drmm_mode_config_init(). No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127212612.77667-1-marex@denx.de