hw/misc/bcm2835_fb: Move config fields to their own struct

hw/misc/bcm2835_fb: Move config fields to their own struct

The handling of framebuffer properties in the bcm2835_property code
is a bit clumsy, because for each of the many fb related properties
we try to track the value we're about to set and whether we're going
to be setting a value, and then we hand all the new values off
to the framebuffer via a function which takes them all as separate
arguments. It would be simpler if the property code could easily
copy all the framebuffer's current settings, update them with
the new specified values and then ask the framebuffer to switch
to the new set.

As the first part of this refactoring, pull all the fb config
settings fields in BCM2835FBState out into their own struct.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180814144436.679-2-peter.maydell@linaro.org