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
4 jobs for xwayland_gamma_workaround in 6 minutes and 40 seconds (queued for 1 second)