BUILTIN, CRTC_C, CLOCK_C, and DEFULT mode types are unused. Let's
refuse to generate them or accept them from userspace either. A
cursory check didn't reveal any userspace code that would depend
on these.

v2: Recommend DRIVER instead of BUILTIN (ajax)

Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Keith Packard <keithp@keithp.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115154504.14338-1-ville.syrjala@linux.intel.comReviewed-by: Jose Abreu <joabreu@synopsys.com>