dev-amdgpu: Add device memory

This adds the actual backing store for the GPU framebuffer.

Change-Id: I22c6dd9bd25b216c4ec99ee472c83d4cb2648efb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57533
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/amdgpu/amdgpu_device.hh

@@ -131,6 +131,11 @@ class AMDGPUDevice : public PciDevice
     // last vmid allocated by map_process PM4 packet
     uint16_t _lastVMID;
 
+    /*
+     * Backing store for GPU memory / framebuffer / VRAM
+     */
+    memory::PhysicalMemory deviceMem;
+
   public:
     AMDGPUDevice(const AMDGPUDeviceParams &p);