intel: use pwrite for batch

It's faster. Not only is the memcpy more efficiently performed in the
kernel (making up for the system call overhead), but by not using mmap
we remove the greater overhead of tracking the vma of every batch.

And it means we can read back from the batch buffer without incurring
the cost of a uncached read through the GTT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/mesa/drivers/dri/intel/intel_fbo.c

@@ -580,7 +580,7 @@ intel_finish_render_texture(struct gl_context * ctx,
     * batch.  Once again, we wish for a domain tracker in libdrm to cover
     * usage inside of a batchbuffer like GEM does in the kernel.
     */
-   intel_batchbuffer_emit_mi_flush(intel->batch);
+   intel_batchbuffer_emit_mi_flush(intel);
 }
 
 /**