video: add guards around 16bpp/32bbp code

Many boards use only single depth configuration, for such boards
there is some unused code in video and console uclass routines.
Add guards to avoid dead code.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Eugen Hristev <eugen.hristev@microchip.com>
Tested-by: Patrice Chotard <patrice.chotard@st.com>
Tested-by: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>