drm/i915: Change the COLOR_ENCODING prop default value to BT.709

Bring us forward from the stone age and switch our default YCbCr->RGB
conversion matrix to BT.709 from BT.601. I would expect most matrial
to be BT.709 these days.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Jyri Sarha <jsarha@ti.com>
Acked-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214192327.3250-8-ville.syrjala@linux.intel.com