drm/i915: Clear out old GT FIFO errors in intel_uncore_early_sanitize()

The BIOS or someone else might have done something bad and there
might be old GT FIFO erros reported in GTFIFODBG. Clear those out
in intel_uncore_early_sanitize() to make sure we don't mistake them
for our problems.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>