radeon: Remove dead STANDALONE_MMIO defines

radeon: Remove dead STANDALONE_MMIO defines

These were, at some point in the past, used to request that Xorg's
compiler.h export a static inline xf86ReadMmio32 instead of a function
pointer. compiler.h only has this option for DEC Alpha.

But Xorg's compiler.h isn't being included by either of these two files
and the radeon driver still works on Alpha, so the definitions are dead
and not needed.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>