drm: add generic driver to drm renderer plugin

drm: add generic driver to drm renderer plugin

Since around 2.6.39, the kernel has offered a generic ioctl interface
for modesetting (the KMS 'dumb' interface).  This interface is now
provided by most all of the modesetting drivers.

Adding support for this interface means that plymouth will automatically
gain support for new modesetting drivers going forward.

This commit adds that support. Some changes made by Ray Strode.