drm/radeon/kms: add support for the CONST IB to the CS ioctl

This adds a new chunk id to the CS ioctl to support the
INDIRECT_BUFFER_CONST packet.

On SI, the CP adds a new engine called the CE (Constant Engine)
which runs simulatenously with the DE (Drawing Engine, formerly
called the ME).  This allows the CP to process two related IBs
simultaneously.  The CE is tasked with loading the constant data
(constant buffers, resource descriptors, samplers, etc.) while
the DE loads context register state and issues drawing commands.
It's up to the userspace application to sychronize the CE and the
DE using special synchronization packets.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>