drm/i915/perf: leave GDT_CHICKEN_BITS programming in configs

There will be a need for userspaces configurations to set this
register. We can apply the same model inside the kernel for test
configs.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170803165812.2373-4-lionel.g.landwerlin@intel.com