drm/amdgpu: align frag_end to covered address space

align frag_end to the next pd when there are no
page table entries on the current pde.
This fixes invalidation of larger address space areas
where some page tables are allocated and other aren't.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>