drm/amdgpu: use correct rounding macro for 64-bit

This fixes 32-bit arm build due to lack of 64-bit divides.

Fixes: cb1c8146 ("drm/ttm: flip the switch for driver allocated resources v2")
Link: https://patchwork.freedesktop.org/patch/438442/


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>