drm/ttm: Avoid conflicting reserve_memtype during ttm_tt_set_page_caching.

Fixes errors like:
> reserve_ram_pages_type failed 0x15b7a000-0x15b7b000, track 0x8, req 0x10
when a BO is moved between WC and UC areas.

Reported-by: Xavier Chantry <shiningxc@gmail.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>