vgabios: Unify X_init() functions.

vgabios: Unify X_init() functions.

Use the same function signature for cirrus, bochsvga, geodelx, and
stdvga init code.

Have each hardware type explicitly call stdvga_init when needed.

To unify bochsvga_init() signature, store the device BDF passed into
the optionrom init code in a global variable (VgaBDF).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>