drm/uapi: Deprecate DRM_MODE_FLAG_BCAST

Reject any mode with DRM_MODE_FLAG_BCAST. We have no code that even
checks for this flag hence it can't possibly do any good.

I think this maybe originated from fbdev where it was supposed to
indicate PAL/NTSC broadcast timings. I have no idea why those would
have to be identified by a flag rather than by just the timings
themselves. And then I assume it got copied into xfree86 for
fbdevhw, and later on it leaked into the randr protocol and kms uapi.

Since kms fbdev emulation never uses the corresponding fbdev flag
there should be no sane way for this to come back into kms via
userspace either.

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114183258.16976-5-ville.syrjala@linux.intel.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>