drm/amdgpu: Add debugfs file for VBIOS and version

Add 2 debugfs files, one that contains the VBIOS version, and one that
contains the VBIOS itself. These won't change after initialization,
so we can add the VBIOS version when we parse the atombios information.

This ensures that we can find out the VBIOS version, even when the dmesg
buffer fills up, and makes it easier to associate which VBIOS version is
for which GPU on mGPU configurations. Set the size to 20 characters in
case of some weird VBIOS version that exceeds the expected 17 character
format (3-8-3\0). The VBIOS dump also allows for easy debugging

    v2: Move to debugfs, clarify commit message, add VBIOS dump file

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