drm/amdgpu: remove inproper workaround for vega10

the workaround is not needed for soc15 ASICs except
for vega10. it is even not needed with latest vega10
vbios.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>