drm/amdgpu: Support setting reset_method at runtime

In order to support more test cases, support user
change reset_method at runtime.

Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>