drm/amd: Explicitly check for GFXOFF to be enabled for s0ix

If a user has disabled GFXOFF this may cause problems for the suspend
sequence.  Ensure that it is enabled in amdgpu_acpi_is_s0ix_active().

The system won't reach the deepest state but it also won't hang.

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