drm/radeon: Add 32bit ioctls support

This allows to run 32bit applications on a 64bit host. This was tested
successfully with Wine (emulators/i386-wine-devel) and StarCraft II.

Before this change, running a 32bit OpenGL application would trigger a
kernel panic.

This is an MFC of r265262.

Submitted by:	Jan Kokemüller <jan.kokemueller@gmail.com>
Approved by:	re (gjb)