console: Clean up bytes per pixel calculation

console: Clean up bytes per pixel calculation

Division with round up is the correct way to compute this even if the
only case where division with round down gives incorrect result is
probably 15 bpp. This case was explicitely patched up in one of these
functions but was unhandled in the other. (I'm not sure about setting
16 bpp for the 15bpp case either but I left that there for now.)

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>