Make the call to intel_update_watermarks() just once or twice during
modeset. Ideally it should happen independently when each plane gets
enabled/disabled, but for now it seems better to keep it in central
place. We can improve things when we get all the planes sorted out
in a better way.

When enabling set up the watermarks just before the pipe is enabled.
And when disabling we need to wait until we've marked the crtc as
inactive, as otherwise intel_crtc_active() would still think the pipe
is enabled and the computed watermarks would reflect that.

v2: Pimp up the commit message a bit
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>