splash_source: rename *_read() to *_read_raw()

Rename raw read functions to *_read_raw() in preparation for supporting
read_fs() feature.

Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>