radeonsi: convert constant buffers to si_descriptors

radeonsi: convert constant buffers to si_descriptors

There is a new "class" si_buffer_resources, which should be good enough for
implementing any kind of buffer bindings (constant buffers, vertex buffers,
streamout buffers, shader storage buffers, etc.)

I don't even keep a copy of pipe_constant_buffer - we don't need it.

The main motivation behind this is to have a well-tested infrastrusture
for setting up streamout buffers.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Tom Stellard <thomas.stellard@amd.com>