video: Fix crash when scroll screen

After enabling log printing to lcd, when the screen starts
scrolling, system crashes. Log is shown as bellow:

    "Synchronous Abort" handler, esr 0x96000045
    "Synchronous Abort" handler, esr 0x96000045

Checking the source code, we found that the variable "pixels"
gets a wrong value:

    int pixels = VIDEO_FONT_HEIGHT * vid_priv->line_length;

"pixels" here means the value of pixels for a character, rather
than the bytes for a character. So the variable "pixels" is 4
times bigger than it's exact value, which will cause the memory
overflow when the cpu runs the following code:

    for (i = 0; i < pixels; i++)
        *dst++ = clr; <<----

Signed-off-by: Eric Gao <eric.gao@rock-chips.com>