drm: fail gracefully when proc isn't setup.

If drm can't find proc it should fail more gracefully, than just
oopsing, this tests drm_class is NULL, and sets it to NULL in the
fail paths.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>