drm/amd: Convert USB-C PD F/W attributes into groups

Rather than special casing the creation of the file, special case
the visibility to the supported dGPUs.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>