hw/misc/bcm2835_property: Track fb settings using BCM2835FBConfig

hw/misc/bcm2835_property: Track fb settings using BCM2835FBConfig

Refactor the fb property setting code so that rather than
using a set of pointers to local variables to track
whether a config value has been updated in the current
mbox and if so what its new value is, we just copy
all the current settings of the fb at the start, and
then update that copy as we go along, before asking
the fb to switch to it at the end.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180814144436.679-3-peter.maydell@linaro.org