drm/amdgpu: set sw state to gfxoff after SR-IOV reset

[Why]
Current SR-IOV will not set GC to off state, while it is a real
GC hard reset. Whthout GFX off flag, driver may do gfxhub invalidation
before firmware load and gfxhub gart enable. This operation may cause
CP to become busy because GC is not in the right state for invalidation.

[How]
Add a function for SR-IOV to clean up some sw state before recover. Set
adev->gfx.is_poweron to false to prevent gfxhub invalidation before gfx
firmware autoload complete.

Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: HaiJun Chang <HaiJun.Chang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>