drm/radeon: Fix wrong boolean operator

This error is reported by cppcheck:
drivers/gpu/drm/radeon/radeon_encoders.c:1066: warning: Mutual exclusion over || always evaluates to true. Did you intend to use && instead?

It looks like cppcheck is correct, so fix this. No test was run.

Cc: David Airlie <airlied@linux.ie>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>