drm/i915: Make sure computed watermarks never overflow the registers

When we calculate the watermarks for a pipe make sure we leave any
level fully zeroed out if it would exceed any of the maximum values
that fit in the registers.

This will be important later when we start to use also disabled
watermark levels during LP1+ merging.

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>