uvesafb: abort initialization if video=uvesafb is not set on cmdline

This patch makes possible to ship kernels with both vesafb and uvesafb
in order to guarantee a smooth transition to uvesafb and cope with
potential incompatibiles introduced by uvesafb making possible to disable
it via cmdline.

In case both vesafb and uvesafb are built-in, the kernel will try to
initialize both, which makes possible to select the wanted one using
either video=vesafb:... or video=uvesafb:....
In this way, old distro installations will keep working as before while
new ones can adopt video=uvesafb.

The behaviour does not change if uvesafb is built as a module.