i965: Allocate VMA in userspace for full-PPGTT systems.

This patch enables soft-pinning of all buffers, allowing us to skip
relocation processing entirely.  All systems with full PPGTT and > 4GB
of VMA should gain these benefits.  This should be most Gen8+.

Unfortunately, this excludes a few systems:
- Cherryview (only has 32-bit addressing, despite 48-bit pointers)
- Broadwell with a 32-bit kernel
- Anybody running pre-4.5 kernel.

We may enable it for Cherryview in the future, but it would require
some tweaks to the memory zone.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_bufmgr.c

@@ -1724,7 +1724,7 @@ brw_bufmgr_init(struct gen_device_info *devinfo, int fd)
       bufmgr->initial_kflags |= EXEC_OBJECT_SUPPORTS_48B_ADDRESS;
 
       /* Allocate VMA in userspace if we have softpin and full PPGTT. */
-      if (false && gem_param(fd, I915_PARAM_HAS_EXEC_SOFTPIN) > 0 &&
+      if (gem_param(fd, I915_PARAM_HAS_EXEC_SOFTPIN) > 0 &&
           gem_param(fd, I915_PARAM_HAS_ALIASING_PPGTT) > 1) {
          bufmgr->initial_kflags |= EXEC_OBJECT_PINNED;