hw/display/bcm2835_fb: Drop unused size and pitch fields

hw/display/bcm2835_fb: Drop unused size and pitch fields

The BCM2835FBState struct has a 'pitch' field which is a
cached copy of xres * (bpp >> 3), and a 'size' field which is
a cached copy of pitch * yres. However we don't actually do
anything with these fields; delete them. We retain the
now-unused slots in the VMState struct for migration
compatibility.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180814144436.679-4-peter.maydell@linaro.org