vk: Reorder gen8 specific code into three new files

vk: Reorder gen8 specific code into three new files

We'll organize gen specific code in three files per gen: pipeline,
cmd_buffer and state, eg:

    gen8_cmd_buffer.c
    gen8_pipeline.c
    gen8_state.c

where gen8_cmd_buffer.c holds all vkCmd* entry points, gne8_pipeline.c
all gen specific code related to pipeline building and remaining state
code (sampler, surface state, dynamic state) in gen8_state.c.

Signed-off-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>