Remove MODULES_PLYMOUTH

MODULES_PLYMOUTH is only useful to mdev and doesn't do any good
to udev because it dynamically loads what's needed
gen_cmdline: force --udev when --plymouth is selected

Plymouth requires KMS, and mdev doesn't do dynamic modules loading,
while udev does.
gen_initramfs: append drm drivers to initramfs if plymouth is enabled

Plymouth activates the drm subsystem through udev in order to setup
its splash screen.