drm/amdgpu: Fix a bug on flag table_freed

table_freed will be always true when mapping a memory with size
bigger than 2MB. The problem is page table's entries are always
existed, but existing mapping depends on page talbe's bo, so
using a check of page table's bo existed will resolve the issue.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>