video: Move BMP pixel-writing into a function

video: Move BMP pixel-writing into a function

At present the code that writes to a pixel is quite convoluted. It uses a
colour map which is in the uclass and the same code is repeated in
different places within video_bmp_display().

As a first step, create a function which can write a pixel from the
bitmap, no matter what the display depth. Use any provided palette
directly, rather than using the uclass version.

Signed-off-by: Simon Glass <sjg@chromium.org>