radeon: Do not directly dereference pointers to BIOS area.

Use readb() and memcpy_fromio() accessors instead.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org