fbcon: Introduce get_{fg,bg}_color()

Make the code more legible by adding get_{fg,bg}_color()
which hide the obscure 'is_fg' parameter of get_color()
from the caller.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Helge Deller <deller@gmx.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>