compulab: splash: refactor splash.c

Move storage device specific code into its own function instead of
calling it directly from an otherwise storage device independent
function (cl_splash_screen_prepare).

This is a preparation for supporting multiple splash locations.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>