drm/amdgpu: Use dev_get_drvdata where possible

Instead of using to_pci_dev + pci_get_drvdata,
use dev_get_drvdata to make code simpler.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>