config: enable CMD_BMP when API+LCD is enabled

When both CONFIG_API and CONFIG_LCD are enabled, the API code calls
lcd_display_bitmap(). That isn't compiled unless either CONFIG_CMD_BMP
or CONFIG_SPLASH_SCREEN is enabled. In order to prevent build problems,
have config_fallbacks.h enable CONFIG_CMD_BMP when both API and LCD are
enabled.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>