drm: define drm_compat_ioctl NULL on CONFIG_COMPAT=n and reduce #ifdefs

If we define drm_compat_ioctl NULL on CONFIG_COMPAT=n, we don't have to
check for the config everywhere.

Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1478014844-27454-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -249,9 +249,7 @@ static const struct file_operations mtk_drm_fops = {
 	.mmap = mtk_drm_gem_mmap,
 	.poll = drm_poll,
 	.read = drm_read,
-#ifdef CONFIG_COMPAT
 	.compat_ioctl = drm_compat_ioctl,
-#endif
 };
 
 static struct drm_driver mtk_drm_driver = {