arch-vega: Rename mismatched source/header files

The files registers.cc, isa.cc, and decoder.cc do not match the header
name. This is a minor cleanup to make development more straightforward.

Change-Id: Ibab18dfe315b0ce84359939b490f8227ea43cac0

src/arch/amdgpu/vega/SConscript

@@ -49,12 +49,12 @@ Source('tlb_coalescer.cc')
 DebugFlag('GPUPTWalker', 'Debug flag for GPU page table walker')
 
 if env['CONF']['TARGET_GPU_ISA'] == 'vega':
-    Source('decoder.cc')
+    Source('gpu_decoder.cc')
     Source('insts/gpu_static_inst.cc')
     Source('insts/instructions.cc')
     Source('insts/op_encodings.cc')
-    Source('isa.cc')
-    Source('registers.cc')
+    Source('gpu_isa.cc')
+    Source('gpu_registers.cc')
 
     Source('insts/sop2.cc')
     Source('insts/sopk.cc')

src/arch/amdgpu/vega/gpu_decoder.cc

@@ -29,9 +29,10 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include "arch/amdgpu/vega/gpu_decoder.hh"
+
 #include <vector>
 
-#include "arch/amdgpu/vega/gpu_decoder.hh"
 #include "arch/amdgpu/vega/insts/gpu_static_inst.hh"
 #include "arch/amdgpu/vega/insts/instructions.hh"
 #include "arch/amdgpu/vega/insts/vop3p.hh"