drm: Restrict drm_mode_set_config_internal to non-atomic drivers

This is another case where we really can't reconstruct a acquire ctx
in any useful fashion because all the callers are legacy drivers. So
like drm_plane_force_disable simply restrict it to non-atomic drivers
so that it's clear we're ok with passing a NULL ctx.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-17-daniel.vetter@ffwll.ch