drm/amdgpu: fix usable gart size calculation

amdgpu_do_test_moves() is failing because of wrong
usable gart size calculation and throwing:

[drm:amdgpu_do_test_moves [amdgpu]] *ERROR* 0000000020bdc9f3 bind failed

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>