drm/i915: Extract wm latency adjustment to its own function

Watermark latency is adjusted in cases when latency is 0us for level
greater than 1, the subsequent levels are disabled. Extract this logic
into its own function.

v2: Pass dev_priv to the extracted function(MattR)

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220818234202.451742-14-radhakrishna.sripada@intel.com