vt: splash: Use splash screen passed from loader

If loader(8) gives use a splash screen to use using the MODINFOMD_SPLASH
type, use it if RB_MUTE is set to "YES".
By design only argb data will be displayed.

Differential Revision: https://reviews.freebsd.org/D45931
Reviewed by: imp, tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG