drm/radeon/kms: cleanup r600 blit code

reorganize the code such that only the primitives (i.e., the functions
that load the CP ring) are hardware specific; dynamically link the
primitives in a (new) pointer structure inside r600_blit at
blit initialization time so that the functions that control the blit
operations can be made common for r600 and evergreen parts

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