drm/amdgpu: fix a error injection failed issue

because "sscanf(str, "retire_page")" always return 0, if application use
the raw data for error injection, it always wrongly falls into "op ==
3". Change to use strstr instead.

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>