drm/amdgpu: Optimize the enablement of GECC

Enable GECC only when the default memory ECC mode or
the module parameter amdgpu_ras_enable is activated.

v2: Add kernel message to remind users explicitly set
    amdgpu_ras_enable=1 before driver loading to enable GECC
    and set amdgpu_ras_enable=0 to disable GECC when GECC is
    currently enabled if needed.

Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>