drm: verisilicon: make vs_dc_platform_driver static

drm: verisilicon: make vs_dc_platform_driver static

The platform_driver struct isn't export and is only used for module
init/exit functions generated by module_platform_driver() macro.

Make it static to prevent namespace pollution.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603180616.TM6qYvIY-lkp@intel.com/
Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260324060806.2047121-1-zhengxingda@iscas.ac.cn