drm: Introduce new DRM_FORMAT_XYUV

drm: Introduce new DRM_FORMAT_XYUV

v5: This is YUV444 packed format same as AYUV, but without alpha,
    as supported by i915.

v6: Removed unneeded initializer for new XYUV format.

v7: Added is_yuv field initialization according to latest
    drm_fourcc format structure initialization changes.

v8: Edited commit message to be more clear about skl+, renamed
    PLANE_CTL_FORMAT_AYUV to PLANE_CTL_FORMAT_XYUV as this format
    doesn't support per-pixel alpha. Fixed minor code issues.

v9: Moved DRM format check to proper place in intel_framebuffer_init.

v10: Changed DRM_FORMAT_XYUV to be DRM_FORMAT_XYUV8888

v11: Fixed rebase conflict, caused by added new formats to drm-tip
     meanwhile.

Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
[vsyrjala: Removed stray tab and sorted the formats differently]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109093916.25858-2-stanislav.lisovskiy@intel.com