Revert "drm/amdgpu/nbio7.4: add hw bug workaround for vega20"

This reverts commit e01f2d41.

VG20 did not require this workaround, as the fix is in the VBIOS.
Leave VG10/12 workaround as some older shipped cards do not have the
VBIOS fix in place, and the kernel workaround is required in those
situations

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>