drm/i915/skl+: add NV12 in skl_format_to_fourcc

Add support of recognizing DRM_FORMAT_NV12 from plane_format
register value.

v2: Added reviewed by tag from Mika Kahola

v3: Added reviewed by from Juha-Pekka Heikkila

v4: Rebased the series
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523245273-30264-4-git-send-email-vidya.srinivas@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -2662,6 +2662,8 @@ static int skl_format_to_fourcc(int format, bool rgb_order, bool alpha)
 	switch (format) {
 	case PLANE_CTL_FORMAT_RGB_565:
 		return DRM_FORMAT_RGB565;
+	case PLANE_CTL_FORMAT_NV12:
+		return DRM_FORMAT_NV12;
 	default:
 	case PLANE_CTL_FORMAT_XRGB_8888:
 		if (rgb_order) {