ANDROID: goldfishfb: Set pixclock = 0

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.

Change-Id: I759bf518bf6683446bc786bf1be3cafa02dd8d42
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>