lcd: support displaying 24bpp BMPs on >= 24bpp LCDs

most todays LCDs support 32bpp e.g. the framebuffer memory is 32bpp
organized.

To support 24bpp BMPs we need to take only 3 byte from the bpp and set
one byte from the FB to 0.

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>