drm/amdgpu: move discovery gfx config fetching

Move it into the fw_info function since it's logically part
of the same functionality.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>