radeonsi: Create CE IB.

radeonsi: Create CE IB.

Based on work by Marek Olšák.

v2: Add preamble IB.

Leaves the load packet in the space calculation as the
radeon winsys might not be able to support a premable.

The added space calculation may look expensive, but
is converted to a constant with (at least) -O2 and -O3.

v3: - Fix code style.
    - Remove needed space for vertex buffer descriptors.
    - Fail when the preamble cannot be created.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>