Softlockup on boot with Cape Verde XT on many kernels

[Federico]

Hi.
I always get a soft lockup when booting since I bought a Cape Verde XT
[Radeon HD 7770/8760 / R7 250X].

In this bug report you can find screenshots of the softlockup and the
kernel panic generated by using softlockup_panic=1

https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1386973

I have this problem with and without nomodeset and I have tested the
kernels that come with Ubuntu 14.04, 14.10 and 15.04 (today's daily image).

The live session shows me the boot menu where I can enter kernel  options
and such, but when booting into a live session it soon prints the
softlockup messages forever.

In order to install catalyst I had to disable the card in the BIOS and boot
into the integrated graphics. Catalyst works perfectly.

MoBo is based on AMD 780G (Radeon HD3200 IGP).

If there's anything I can do to help find a workaround or a fix for this,
let me know.
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