drm/amdgpu: Fix GTT size reporting in amdgpu_ioctl

The commit below changed the TTM manager size unit from pages to
bytes, but failed to adjust the corresponding calculations in
amdgpu_ioctl.

Fixes: dfa714b8 ("drm/amdgpu: remove GTT accounting v2")
Bug: drm/amd#1930
Bug: mesa/mesa#6642


Tested-by: Martin Roukala <martin.roukala@mupuf.org>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.18.x