gpu-compute: Add mmap functionality to GPURenderDriver

dGPUs mmap the GPURenderDriver, however it doesn't appear that they do
anything with it. This patch implements the mmap function by just
returning the address provided, while not doing anything else

Change-Id: Ia010a2aebcf7e2c75e22d93dfb440937d1bef3b1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47523
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/gpu-compute/gpu_render_driver.hh

@@ -50,6 +50,9 @@ class GPURenderDriver final : public EmulatedDriver
     {
         return -EBADF;
     }
+
+    Addr mmap(ThreadContext *tc, Addr start, uint64_t length,
+              int prot, int tgt_flags, int tgt_fd, off_t offset) override;
 };
 
 } // namespace gem5