Staging: udlfb: explicit dependencies and warnings

Specify Kconfig dependencies, and include warnings for building as a module

udlfb is dependent on FB_DEFERRED_IO, FB_SYS_*, and FB_MODE_HELPERS

Because many kernels do not include defio (which cannot be built
as a module), yet users want to be able to build udlfb as a module later,
udlfb has ifdefs and these dependency warnings to help udlfb build with or
without certain dependencies, but also print warnings for any lost function.

Even though this kind of flexibility isn't common, we've gotten feedback
from a significant portion of users that they were frustrated without it.

Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>