hw/display/bcm2835_fb: Abstract out calculation of pitch, size

hw/display/bcm2835_fb: Abstract out calculation of pitch, size

Abstract out the calculation of the pitch and size of the
framebuffer into functions that operate on the BCM2835FBConfig
struct -- these are about to get a little more complicated
when we add support for virtual and physical sizes differing.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180814144436.679-6-peter.maydell@linaro.org