drm/amdgpu/nbio2.3: don't use GPU_HDP_FLUSH bit 12

It's used internally by firmware.  Using it in the driver
could conflict with firmware.

v2: squash in fix for navi1x (Alex)

Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>