fbsplash: limit progress bar flicker

fbsplash: limit progress bar flicker

Progress bar updates flicker quite a bit on slow hw / high resolutions
as the background is completely cleared before the new progress bar
position is drawn on top.

Improve it by first drawing the progress bar and then only fill the
remaining rows with the background.

function                                             old     new   delta
fb_drawprogressbar                                   444     429     -15

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>