drm/amdgpu: Increase potential product_name to 64 characters

Having seen at least 1 42-character product_name, bump the number up to
64, and put that definition into amdgpu.h to make future adjustments
simpler.

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>