drm/i915/skl+: NV12 related changes for WM

NV12 requires WM calculation for UV plane as well.
UV plane WM should also fulfill all the WM related restrictions.

v2: Addressed review comments from Shashank Sharma.

v3: Addressed review comments from Shashank Sharma
Changed plane_num to plane_id in skl_compute_plane_wm_params
and skl_compute_plane_wm.
Adding reviewed by tag from Shashank Sharma

v4: Added reviewed by from Juha-Pekka Heikkila

v5: Rebased the series

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-6-git-send-email-vidya.srinivas@intel.com