drm/amdgpu: Fix RLC safe mode test in gfx_v9_0_enter_rlc_safe_mode

We were testing the register offset, instead of the value stored in the
register, therefore always timing out the loop.

This reduces suspend time of the system in the bug report below by ~600
ms.

Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/107277


Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>