sna: Add XV_COLORSPACE attribute support for sprite Xv adaptors

Use the new "COLOR_ENCODING" plane property to implement the
XV_COLORSPACE port attribute for sprite Xv adaptors.

v2: assert(colorspace < ARRAY_SIZE) (Chris)

Cc: Jyri Sarha <jsarha@ti.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>