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)
Status Job ID Name Coverage
  Docker Image
passed #3427591
debian-testing

00:00:41

 
  Build And Test
passed #3427592
autotools

00:05:59

passed #3427593
meson

00:01:13

passed #3427594
meson-noglamor

00:01:04