drm/ttm: Rework validation & memory space allocation (V3)

This change allow driver to pass sorted memory placement,
from most prefered placement to least prefered placement.
In order to avoid long function prototype a structure is
used to gather memory placement informations such as range
restriction (if you need a buffer to be in given range).
Range restriction is determined by fpfn & lpfn which are
the first page and last page number btw which allocation
can happen. If those fields are set to 0 ttm will assume
buffer can be put anywhere in the address space (thus it
avoids putting a burden on the driver to always properly
set those fields).

This patch also factor few functions like evicting first
entry of lru list or getting a memory space. This avoid
code duplication.

V2: Change API to use placement flags and array instead
    of packing placement order into a quadword.
V3: Make sure we set the appropriate mem.placement flag
    when validating or allocation memory space.

[Pending Thomas Hellstrom further review but okay
from preliminary review so far].

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>