drm/mcde: Fix an uninitialized variable

We never set "vblank" to "false".

Current versions of GCC will initialize it to zero automatically at
certain optimization levels so that's probably why this didn't show up
in testing.

Fixes: 5fc537bf

 ("drm/mcde: Add new driver for ST-Ericsson MCDE")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190529113458.GG19119@mwanda