drm/i915: Add missing NULL check before calling initial_watermarks

drm/i915: Add missing NULL check before calling initial_watermarks

Not all platforms set this callback, so NULL check it before calling it.

v2:
- Call intel_update_watermarks() on HSW+ where the callback is not set.
  (Matt)

CC: Matt Roper <matthew.d.roper@intel.com>
Fixes: commit ed4a6a7ca853 ("drm/i915: Add two-stage ILK-style watermark programming (v11)")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456776633-3401-1-git-send-email-imre.deak@intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>