drm/amdgpu: use string choice helpers

Use string choice helpers for better readability.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/r/202410161814.I6p2Nnux-lkp@intel.com/


Signed-off-by: R Sundar <prosunofficial@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>