drm/i915: Add glk to intel_detect_preproduction_hw()

Detect GLK pre-production steppings. Not 100% of A2 being pre-prod
since the spec is a bit of a mess but feels more or less correct.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128155152.21977-4-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>