lcd: cleanup unused functions

This patch removes following two functions:
- lcd_getbgcolor(...)
  not used somewhere outside lcd.c, internally we use now the global
  variable lcd_color_bg (was return value of function before)
- lcd_getfgcolor(...)
  not used in any place of u-boot

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>
[agust: rebased]
Signed-off-by: Anatolij Gustschin <agust@denx.de>