video: bmp: Add support for 24bpp BMP files on 16bpp displays

This patch adds support to load 24bpp BMP files on 16bpp displays. This
will be used by the theadorable board. The "old" bmp command did support
this operartion mode and to not break compatibility with the move to
DM_VIDEO, we need to add this support to the "new" bmp code.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>