drm/i915: Just clear the mmiodebug before a register access

drm/i915: Just clear the mmiodebug before a register access

When we enable the per-register access mmiodebug, it is to detect which
access is illegal. Reporting on earlier untraced access outside of the
mmiodebug does not help debugging (as the suspicion is immediately put
upon the current register which is not at fault)!

References: https://bugs.freedesktop.org/show_bug.cgi?id=97985
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: stable@vger.kernel.org
Link: http://patchwork.freedesktop.org/patch/msgid/20161003124516.12388-1-chris@chris-wilson.co.uk