drm/radeon/kms: simplify evergreen blit code

Covert 4k pages to multiples of 64x64x4 tiles.
This is also more efficient than a scanline based
approach from the MC's perspective.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>