video: fbdev: Set pixclock = 0 in goldfishfb

User space Android code identifies pixclock == 0 as a sign for emulation
and will set the frame rate to 60 fps when reading this value, which is
the desired outcome.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Roman Kiryanov <rkir@google.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>