drm/amdgpu: Apply poison mode check to GFX IP only

For GFX IP that only supports poison consumption, GFX
RAS won't be marked as enabled. i.e., hardware doesn't
support gfx sram ecc. But driver still needs to issue
firmware to enable poison consumption mode for GFX IP.
In such case, check poison mode and treat GFX IP as
RAS capable IP block.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>