drm/i915: Nuke intel_atomic_legacy_gamma_set

We do set DRIVER_ATOMIC now.

Note that the comment is outdated, the property paths switched over to
checking drm_drv_uses_atomic_modeset() a while ago. Which means this
can't even break if we revert DRIVER_ATOMIC again.

v2: Add note that this is even safer (Maarten).

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403083304.9083-9-daniel.vetter@ffwll.ch