drm/amdgpu/gmc6-8: properly disable the AGP aperture

The BOT register needs to be larger than the TOP register
for this to be properly disabled.  The lower 22 bits
of the BOT address are always 0 and the lower 22 bits of
the TOP register are always 1 so you need to make
the upper bits of BOT larger than the upper bits of BOT.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>