drm/i915: Skip watermark merging for inactive pipes

drm/i915: Skip watermark merging for inactive pipes

Even though the inactive pipes should have their watermarks set to all 0
with enable=true, we can possibly shave off a few cycles by completely
skipping the merge procedure for inactive pipes.

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