Fix build with CONFIG_NOUVEAU_PLATFORM_DRIVER enabled:

../drivers/gpu/drm/nouveau/nouveau_platform.c: In function ‘nouveau_platform_probe’:
../drivers/gpu/drm/nouveau/nouveau_platform.c:29:13: error: unused variable ‘ret’ [-Werror=unused-variable]
   29 |         int ret;
      |             ^~~

Fixes: 961ae5f9 ("drm/nouveau: handle pci/tegra drm_dev_{alloc, register} from common code")
Cc: Ben Skeggs <bskeggs@nvidia.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240729123624.868907-1-jani.nikula@intel.com