drm/amdgpu: Use default reset method handler

When reset method is not passed in reset context, look for the handler
for default reset method. On Aldebaran, default reset method for SOCs
connected to CPU over XGMI is MODE2.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Tested-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>