drm/amd/amdgpu: export vbios information (v2)

Allows userspace components to fetch information
from the vbios image.

v2: agd: fix warning

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian Koenig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>