[AMDGPU] Adjust number of SGPRs available in Calling Convention

This reduces the number of SGPRs due to some concerns about running
out of SGPRs if you make all the SGPRs that aren't reserved available
for the calling convention.

Change-Id: Idb4ca4dc72f5b6808cb524ff7270915a8de5b4c1
llvm-svn: 370215