Is it necessary to separate DC power cables and data cables?
The answer is "It all depends".


What is the load on the DC?  If it's very noisy inductive loads, you're going to have noise on the DC line, and it might be considerably more than you'd think
What is the signalling rate on the data lines?  Faster rates are vastly much more sensitive
[EDIT] What line encoding have you got?  Anything differential, such as RS-485, is going to be a lot more robust than something voltage-based, such as RS-232
What coding scheme are you using?  If you have got any scheme with error detection, perhaps it will be ok
What happens if there are errors on the line?  If it is updating a clock display, with the effect of a little time skew that's different to dropping heavy machinery onto workers.


Having said all that, it is quite common to have signal and DC power adjacent.  I have quite a lot of underwater telemetry where we use specially-made DC-power and twisted pair cable for 24 VDC and 250 Kbit/s RS-485.  In another much more noisy environment we use 9600 bit/sec.  Per commenters, of course power-over-ethernet is one of the best examples of high-speed, long-distance, high-power DC and data in the same cable.  (Long and high compared to for example USB or a bus on a PCB.  100 metres, 12 Watts.)

In short: it's perfectly doable, but pay good attention.