drm/uapi: Deprecate DRM_MODE_PROP_PENDING

drm/uapi: Deprecate DRM_MODE_PROP_PENDING

DRM_MODE_PROP_PENDING is not used anywhere (except printed out
by libdrm proptest/modetest).

This seems to be yet another thing blindly copied from xrandr.
Quoting from the protocol spec:
"If 'pending' is TRUE, changes made to property values with
 RRChangeOutputProperty will be saved in the pending property value
 and be automatically copied to the current value on the next
 RRSetCrtcConfig request involving the named output. If 'pending' is
 FALSE, changes are copied immediately."

So it was some kind of early idea for atomic property updates.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306164849.2862-4-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>