drm/amdgpu: complete gfxoff allow signal during suspend without delay

change guarantees that gfxoff is allowed before moving further in
s2idle sequence to add more reliablity about gfxoff in amdgpu IP's
suspend flow

Signed-off-by: Harsh Jain <harsh.jain@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>