xf86: Accept devices with the kernel's ofdrm driver

Add a workaround to accept devices of the kernel's ofdrm driver.
Makes Xorg work on Open Firmware's pre-configured display with the
DRM graphics stack.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
9 jobs for ofdrm in 3 minutes and 57 seconds (queued for 3 seconds)