video: bmp: support 8bits BMP drawing on 24/32 bpp framebuffer

Update video bmp code so that we can display 8 bits logo on
24 or 32 bpp framebuffer.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> # bpi-m1+, bpi-m64