drm/i915: Add debugfs hooks for messign with watermark latencies

Add a few new debugfs files which allow changing the watermark memory
latency values during runtime. This can be used to determine the if the
original BIOS provided latency values are no good.

v2: Drop superfluous plane name from output
    Take modeset locks around the latency value read/write

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>