drm/i915: Update less state during modeset.

No need to repeatedly call update_watermarks, or update_fbc.
Down to a single call to update_watermarks in .crtc_enable

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>