r600,compute: Reserve vtx 3 for kernel arguments
r600,compute: Reserve vtx 3 for kernel arguments

Using vtx 0 does not work for dynamic offsets.

v2: add explanatory comment

Signed-off-by: Jan Vesely <ed664c4bb5cadd95cb7521d1190a56e280fa6954@rutgers.edu>
Reviewed-by: Tom Stellard <85590917cb30a670bf23f5b4273617f8815d1e87@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler