drm/radeon/kms: use linear aligned for 6xx/7xx bo blits

Not only is linear aligned supposedly more performant,
linear general is only supported by the CB in single
slice mode.  The texture hardware doesn't support
linear general, but I think the hw automatically
upgrades it to linear aligned.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>