drm/tilcdc: Fix an incorrect condition

Instead of checking if num_encoders is zero, it is being assigned 0.
Convert the assignment to a check.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>