drm/amdgpu: remove unnecessary asic check for sdma5.2

For sdma5.2, all sdma instances will share the same fw,
remove unnecessary asic check to be more generic.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>