csputils: add DCI-P3 colorspace

This colorspace has been historically used as a calibration target for
most digital projectors and sees some involvement in the UltraHD
standards, so it's a useful addition to mpv.

video/csputils.h

@@ -63,6 +63,7 @@ enum mp_csp_prim {
     MP_CSP_PRIM_ADOBE,
     MP_CSP_PRIM_PRO_PHOTO,
     MP_CSP_PRIM_CIE_1931,
+    MP_CSP_PRIM_DCI_P3,
     MP_CSP_PRIM_COUNT
 };