[AMDGPU] Provide control to force VGPR MFMA form (llvm#148079)

This gives an override to the user to force select VGPR form of MFMA.
Eventually we will drop this in favor of compiler making better
decisions, but this provides a mechanism for users to address the cases
where MayNeedAGPRs favors the AGPR form and performance is degraded due
to poor RA.

Author: jrbyrnes