drm/radeon: add 2-level VM pagetables support v9

PDE/PTE update code uses CP ring for memory writes.
All page table entries are preallocated for now in alloc_pt().

It is made as whole because it's hard to divide it to several patches
that compile and doesn't break anything being applied separately.

Tested on cayman card.

v2: rebased on top of "refactor set_page chipset interface v3",
    code cleanups

v3: switched offsets calc macros to inline funcs where possible,
    remove pd_addr from radeon_vm, switched RADEON_BLOCK_SIZE define,
    to 9 (and PTE_COUNT to 1 << BLOCK_SIZE)

v4 (ck): move "incr" documentation to previous patch, cleanup and
         document RADEON_VM_* constants, change commit message to
         our usual format, simplify patch allot by removing
         everything current not necessary, disable SI workaround.

v5: (agd5f): Fix typo in tables_size calculation in
             radeon_vm_alloc_pt().  Second line should have been
             '+=' rather than '='.

v6: fix npdes calculation. In scenario when pfns to be mapped overlap
two PDE spans:

   +-----------+-------------+
   | PDE span  | PDE span    |
   +-----------+----+--------+
          |         |
          +---------+
          | pfns    |
          +---------+

the following npdes calculation gives incorrect result:

npdes = (nptes >> RADEON_VM_BLOCK_SIZE) + 1;

For the case above picture it should give npdes = 2, but gives one.

This patch corrects it by rounding last pfn up to 512 border,
first - down to 512 border and then subtracting and dividing by 512.

v7: Make npde calculation clearer, fix ndw calculation.

v8: (agd5f): reserve enough for 2 full VM PTs, add some
             additional comments.

v9: fix typo in npde calculation

Signed-off-by: Dmitry Cherkasov <Dmitrii.Cherkasov@amd.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>