lcd: introduce getters for bg/fg color

Introduce lcd_getbgcolor() and lcd_getfgcolor(), and use them where
applicable.

This is a preparatory step for extracting lcd console code into its own
file.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>