Err, no. To preserve your DRM keys you need a way to root without unlocking the bootloader (e.g. using an exploit), then you make a backup of your TA partition which will contain your DRM keys. If you unlocked your bootloader without a taking a TA backup first, your DRM keys are gone forever, even if you root your device afterwards and make a backup of your TA partition you won't have any keys at all because they already were wiped when you unlocked the bootloader, this applies to any Sony device.
In other words, currently there's no way of unlocking M5 bootloader and keeping your DRM keys, the hope is Xperia Z5 series or other flagship devices, they draw attention of many developers and like what happened with Z2 and Z3, for example, locked bootloader users probably gonna start a bounty or some kind of incentive for a way to root the locked variant of those flagships, eventually they'll find an exploit that probably can be ported to other devices with less development activity, like Xperia M4/M5.
It's the cat-and-mouse game, new flagships are released, devs will research possible exploits to those new flagship devices and when one is found it's very likely to work with most devices released at the same time (and that probably are unpatched too). Also, in Sony devices, if you have root in one firmware version, you'll automatically have root in any new firmware too (using tools like PRFCreator or dd Flasher)...
(as record, towelroot, developed originally for Galaxy S5, could be used in a lot of Xperia devices from that time, and the Xperia Z3 exploit was successfully ported to Xperia E3 as well and could probably be used in similar devices released around the same time like Xperia C3 and others)
