gallium: Add a util_blitter path for using a custom VS and FS.

gallium: Add a util_blitter path for using a custom VS and FS.

Like the r600 paths to use other custom states, we pass in a couple of
parameters to customize the innards of the blitter.  It's up to the caller
to wrap other state necessary for its shaders (for example, constant
buffers for the uniforms the shader uses).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>