[AMDGPU] Forward-declare AMDGPUTargetMachine (NFC)

AMDGPUTargetTransformInfo.h needs AMDGPUTargetMachine but relies on a
forward declaration of AMDGPUTargetMachine in AMDGPU.h.  This patch
adds a forward declaration right in AMDGPUTargetTransformInfo.h.

While we are at it, this patch removes the one in
AMDGPU.h, where it is unnecessary.