drm/radeon: Fix memory allocation failures in the preKMS command stream checking.

Allocation of single large block of memory may fail under memory
presure. drm_buffer object can hold one large block of data in
multiple independ pages which preents alloation failures.

This patch converts all access to command stream to use drm_buffer
interface. All direct access to array has to go tough drm_buffer
functions to get correct pointer.

Outputting the command stream to ring buffer needs to be awear of
the split nature of drm_buffer. The output operation requires the
new OUT_RING_DRM_BUFFER.

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>