I think regs[IMC] = 0xffffffff is correct.

If I understand this correctly, regs[IMC] = 0xffffffff will clear all bits in the Interrupt Mask Set/Read (IMS) register while 
regs[IMC] = 0 leaves the bits in IMS register unchanged (btw IMS register is not defined in the file probably because interrupts are unused). 

Now, the doc says that initially all bits (except bit 18 - MNG, bit 19 - Reserved) of IMS register are 0 which means no interrupt except for the MNG/Reserved ones (whose inital values are unknown) are enabled by default (Probably that explains why it was working even without this change ;-) )