drm/amdgpu: Fix error handling in amdgpu_ras_recovery_init

Don't set a struct pointer to NULL before freeing its members. It's
hard to see what's happening due to a local pointer-to-pointer data
aliasing con->eh_data.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Tested-by: Philip Cox <Philip.Cox@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>