The vesa framebuffer is only used by architectures that explicitly
require it (i.e. x86). Compile it out for architectures not using it, as
its current implementation might not work for them.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>