Prevent NULL pointer dereference in case when radeon_ring_fini() did it's job.

Reading of r100_cp_ring_info and radeon_ring_gfx debugfs entries will lead to a KP if ring buffer was deallocated, e.g. on failed ring test.
Seen on PA-RISC machine having "radeon: ring test failed (scratch(0x8504)=0xCAFEDEAD)" issue.

v2: agd5f: add some parens around ring->ready check
Signed-off-by: Alex Ivanov <gnidorah@p0n4ik.tk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org