drm/radeon: Enable DMA_IB_SWAP_ENABLE on big endian hosts.

Fixes GPU hang during DMA ring IB test.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59672



Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>