drm/msm/dpu: Initialize return value for dpu_assign_plane_resources()

Initialize the return value so that the dpu_crtc_atomic_check() doesn't
fail if the virtual planes command line parameter is enabled and no planes
are visible.

Fixes: 774bcfb7 ("drm/msm/dpu: add support for virtual planes")
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>