anv/cmd_buffer: Add a new genX_cmd_buffer file for shared code

anv/cmd_buffer: Add a new genX_cmd_buffer file for shared code

This file contains code that can be shared across gens modulo recompiling.
In particular, we can share STATE_BASE_ADDRESS setup and handling of the
vkPipelineBarrier call.  Not sharing STATE_BASE_ADDRESS setup has already
been a source of bugs and the gen7 and gen8 implementations of
PipelineBarrier were line-for-line identical.

Incidentally, this should fix MOCS settings for dynamic and surface state
on Haswell.