drm/radeon: Add 32bit ioctls

This allows to run 32bit applications on a 64bit host.

This is tested successfully with Wine (i386-wine-devel) and StarCraft
II. This test required a recompiled i386-wine-devel package to include
WITH_GALLIUM in graphics/dri (not enabled by default), and the manual
installation of libtxc_dxtn [1] (dlopened by Mesa).

[1] https://github.com/divVerent/s2tc

Submitted by:	Jan Kokemüller <jan.kokemueller@gmail.com> (previous version)