drm/amd: make amdgpu_ucode_validate static

No consumers outside of amdgpu_ucode.c use amdgpu_ucode_validate
anymore, so make the function static.

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