drm/amdgpu: use gpu virtual address for interrupt packet write space for vangogh

The interrupts are not stable while uses guest physical address (GPA)
for interrupt packet write space even on direct loading case.

v2: make condition more readable

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>