drm/i915: Split watermark level computation from the code

Refactor the watermarks computation for one level to a separate
function. This function will now set the ->enable flag to true,
even if the watermark level wasn't actually checked yet. In the
future we will delay the checking so we must consider all unchecked
watermarks as possibly valid.

v2: Preserve comment about latency units

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>