drm/radeon/kms: common definitions for blit copy code

R600/700 and Evergreen/NI blit code have a few redundant
definitions in respective .c file. Move common definitions
into a separate (new) .h file.

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