drm/ttm: Work around performance regression with VM_PFNMAP
drm/ttm: Work around performance regression with VM_PFNMAP

A performance regression was introduced in TTM in linux 3.13 when we started using
VM_PFNMAP for shared mappings. In theory this should've been faster due to
less page book-keeping but it appears like VM_PFNMAP + x86 PAT + write-combine
is a particularly cpu-hungry combination, as seen by largely increased
cpu-usage on r200 GL video playback.

Until we've sorted out why, revert to always use VM_MIXEDMAP.
Reference: freedesktop.org bugzilla bug #75719

Reported-and-tested-by: <034fdde4dc3017627fc5e1fefc075a7eee655c8c@gmail.com>
Acked-by: Alex Deucher <08dc22c6156113f2deff178e35e3ed9b24d6af9e@amd.com>
Signed-off-by: Thomas Hellstrom <a26490dfbee4be3d38287ad6448444504f58193c@vmware.com>
Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs