drm/amdgpu: Do not directly dereference pointers to BIOS area.

Use readb() and memcpy_fromio() accessors instead.

Ported from radeon commit:
f2c9e560

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>