drm/radeon: remove unecessary assignment

length_dw field was assigned twice. While at it, move user_ptr
assignment together with all other assignments to p->chunks[i]
structure to make the code more readable.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>