common: Enable splash functions at SPL

To support splash screen at both u-boot proper and SPL use
CONFIG_IS_ENABLED and CONFIG_VAL to check for video related Kconfigs at
respective stages.

Replace CONFIG_CMD_BMP with CONFIG_BMP to enable splash_display function
at u-boot proper and SPL.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>
Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>