[GPU] Cleanup definitions of some registers

VS/PS_NUM_REG is 6-bit on Adreno 200, and games aren't seen using the
bit 7 to indicate that no GPRs are used. It's not clear why Freedreno
configures it this way.

Some texture fetch fields were deprecated or moved during the development
of the Xenos, reflect that in the comments.

Add definitions of the registers configuring the conversion of vertex
positions to fixed-point. Although there isn't much that can be done with
it when emulating using PC GPU APIs, there are some places in Xenia that
wrongly (though sometimes deliberately, for results closer to the behavior
of the host GPU) assume that the conversion works like in Direct3D 10+,
however the Xenos supports only up to 4 subpixel bits rather than 8. The
effects of this difference are largely negligible, though.

Also add more detailed info about register references and differences from
other ATI/AMD GPUs for potential future contributors.

src/xenia/gpu/vulkan/vulkan_command_processor.cc

@@ -2159,8 +2159,8 @@ bool VulkanCommandProcessor::IssueDraw(xenos::PrimitiveType prim_type,
 
   const RegisterFile& regs = *register_file_;
 
-  xenos::ModeControl edram_mode = regs.Get<reg::RB_MODECONTROL>().edram_mode;
-  if (edram_mode == xenos::ModeControl::kCopy) {
+  xenos::EdramMode edram_mode = regs.Get<reg::RB_MODECONTROL>().edram_mode;
+  if (edram_mode == xenos::EdramMode::kCopy) {
     // Special copy handling.
     return IssueCopy();
   }
@@ -2192,9 +2192,9 @@ bool VulkanCommandProcessor::IssueDraw(xenos::PrimitiveType prim_type,
       draw_util::IsRasterizationPotentiallyDone(regs, primitive_polygonal);
   VulkanShader* pixel_shader = nullptr;
   if (is_rasterization_done) {
-    // See xenos::ModeControl for explanation why the pixel shader is only used
+    // See xenos::EdramMode for explanation why the pixel shader is only used
     // when it's kColorDepth here.
-    if (edram_mode == xenos::ModeControl::kColorDepth) {
+    if (edram_mode == xenos::EdramMode::kColorDepth) {
       pixel_shader = static_cast<VulkanShader*>(active_pixel_shader());
       if (pixel_shader) {
         pipeline_cache_->AnalyzeShaderUcode(*pixel_shader);