drm/amd: Flush GFXOFF requests in prepare stage

If the system hasn't entered GFXOFF when suspend starts it can cause
hangs accessing GC and RLC during the suspend stage.

Cc: <stable@vger.kernel.org> # 6.1.y: 5095d541 ("drm/amd: Evict resources during PM ops prepare() callback")
Cc: <stable@vger.kernel.org> # 6.1.y: cb11ca32 ("drm/amd: Add concept of running prepare_suspend() sequence for IP blocks")
Cc: <stable@vger.kernel.org> # 6.1.y: 2ceec37b ("drm/amd: Add missing kernel doc for prepare_suspend()")
Cc: <stable@vger.kernel.org> # 6.1.y: 3a9626c8 ("drm/amd: Stop evicting resources on APUs in suspend")
Cc: <stable@vger.kernel.org> # 6.6.y: 5095d541 ("drm/amd: Evict resources during PM ops prepare() callback")
Cc: <stable@vger.kernel.org> # 6.6.y: cb11ca32 ("drm/amd: Add concept of running prepare_suspend() sequence for IP blocks")
Cc: <stable@vger.kernel.org> # 6.6.y: 2ceec37b ("drm/amd: Add missing kernel doc for prepare_suspend()")
Cc: <stable@vger.kernel.org> # 6.6.y: 3a9626c8 ("drm/amd: Stop evicting resources on APUs in suspend")
Cc: <stable@vger.kernel.org> # 6.1+
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3132
Fixes: ab475033

 ("drm/amdgpu/sdma5.2: add begin/end_use ring callbacks")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>