drm/uapi: Deprecate DRM_MODE_FLAG_PIXMUX

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

Looks like this flag had something to do the the controller<->ramdac
interface with some ancient S3 graphics adapters. Why someone though
it would be a good idea to expose it directly to users I don't know.
And later on it got copied into the randr protocol and kms uapi.

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/20171114183258.16976-4-ville.syrjala@linux.intel.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>