drm/i915: Pass crtc to intel_update_watermarks()

Passing the appropriate crtc to intel_update_watermarks() should help
in avoiding needless work in the future.

v2: Avoid clash with internal 'crtc' variable in some wm functions

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>