drm/amdgpu: Fix vbios build number parsing logic

It's not necessary that the build string and atom header section has a
difference of 32 bytes. Use the remaining bytes in the section as copy
limit.

Fixes: d6fa8026 ("drm/amdgpu: Add vbios build number interface")
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>