drm/amdgpu: clean up discovery testing

Rather than checking of the variable is enabled and the
chip is the right family check for the presence of the
discovery table.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>