drm/msm/sde: modify check for active datapaths

In some cases like suspend and cwb concurrencies,
the number of mixers in the sde_crtc structure
can become zero. Add support to get the number
of mixers from topology in those cases to
avoid incorrect resource allocation request.

Change-Id: Id9b82e805ff50a107ad06514b4e41c0917abdf33
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>