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