lcd: introduce lcd_set_cmap

Reduce the lcd_display_bitmap #ifdef complexity by extracting Atmel-specific
code for setting cmap for bitmap images into a new function lcd_set_cmap().
A default version is implemented with the remainder of the code.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Cc: Bo Shen <voice.shen@atmel.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Anatolij Gustschin <agust@denx.de>