Change the use of a reserved color space entry

This commit rename a reserved color space entry to BT_2020, it intends
to provide support for VP9 bitstream to pass along the color space
type defined in BT.2020(Rec.2020)

please note this entry does not have any effect on encoding/decoding
behavior, but allow applications to the pass the information along
from encoding end to decoding end.

Change-Id: I4678520e89141ea5e8900f7bd1c0e95b710b7091

vp9/common/vp9_enums.h

@@ -104,7 +104,7 @@ typedef enum {
   BT_709     = 2,  // YUV
   SMPTE_170  = 3,  // YUV
   SMPTE_240  = 4,  // YUV
-  RESERVED_1 = 5,
+  BT_2020    = 5,  // YUV
   RESERVED_2 = 6,
   SRGB       = 7   // RGB
 } COLOR_SPACE;