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