xwayland: Initialise values in xwlVidModeGetGamma()

ProcVidModeGetGamma() relies on GetGamma() to initialise values if it
returns TRUE. Without this, we're sending uninitialised values to
clients.

Fixes: xorg/xserver#1040
(cherry picked from commit 6748a40941)

hw/xwayland/xwayland-vidmode.c

@@ -332,6 +332,7 @@ static Bool
 xwlVidModeGetGamma(ScreenPtr pScreen, float *red, float *green, float *blue)
 {
     /* Unsupported for now, but pretend it works */
+    *red = *green = *blue = 1.0f;
     return TRUE;
 }