drm/amd/amdgpu: Enable MMHUB prefetch for ISP v4.1.0 and 4.1.1

Remove temporary WA to disable ISP prefetch as MMHUB SAW is initialized
to support ISP HW access GART memory using the TLSi path with prefetch
enabled.

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