Revert "drm/amdgpu: Fix GTT size reporting in amdgpu_ioctl"

This reverts commit ba4ec352.

This patch is only relevant to kernels which contain
commit dfa714b8 ("drm/amdgpu: remove GTT accounting v2").
amd-staging-drm-next does not.

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