Clearing the watermarks for all pipes/planes when updating the
watermarks for a single CRTC change seems like the wrong thing to
do here. As is, this code will ony update any pipe/plane watermarks
that need updating and leave the remaining set to zero.  Later, the
watermark checks in check_wm_state() will flag these zero'd out pipe/plane
watermarks and throw errors.

By clearing only the watermark values associated with the specific crtc
the other watermark values may remain unchanged.

v2: Make sure all the dirty flags are cleared. Damien
    Clear all values assoicated with crtc/pipe being updated.  Damien
Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>