drivers: video: sw_generator: modify video_sw_generator_fill_colorbar()

Add a check for the array size to avoid overwriting unrelated memory when
the buffer is too small for the full format. It first check if there is
enough buffer for one line, and fill it programmatically. Then, it will
try to duplicate that line over the entire buffer, in the limit of the
room available.

Signed-off-by: Josuah Demangeon <me@josuah.net>