drm/i915/skl+: support verification of DDB HW state for NV12

For YUV 420 Planar formats like NV12,
buffer allocation is done for Y and UV surfaces separately.
For NV12 plane formats, the UV buffer
allocation must be programmed in the Plane Buffer Config register
and the Y buffer allocation must be programmed in the
Plane NV12 Buffer Config register. Both register values
should be verified during verify_wm_state.

v2: Addressed review comments by Maarten.

v3: Addressed review comments by Shashank Sharma.

v4: Adding reviewed by tag from Shashank Sharma

v5: Added reviewed by from Juha-Pekka Heikkila

v6: 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>
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/1523245273-30264-5-git-send-email-vidya.srinivas@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -2657,7 +2657,7 @@ static int i9xx_format_to_fourcc(int format)
 	}
 }
 
-static int skl_format_to_fourcc(int format, bool rgb_order, bool alpha)
+int skl_format_to_fourcc(int format, bool rgb_order, bool alpha)
 {
 	switch (format) {
 	case PLANE_CTL_FORMAT_RGB_565: