drm/radeon: Fix size overflow

ttm->num_pages is uint32. Hit overflow when << PAGE_SHIFT directly

Fixes: 230c079f ("drm/ttm: make num_pages uint32_t")
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>