drm/amdgpu: remove duplicate function prototypes

There are two identical function prototypes in same header file
Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -1680,7 +1680,6 @@ static inline struct amdgpu_device *amdgpu_ttm_adev(struct ttm_bo_device *bdev)
 	return container_of(bdev, struct amdgpu_device, mman.bdev);
 }
 
-bool amdgpu_device_is_px(struct drm_device *dev);
 int amdgpu_device_init(struct amdgpu_device *adev,
 		       struct drm_device *ddev,
 		       struct pci_dev *pdev,