drm/atomic-helper: update lut props directly in ..._legacy_gamma_set

Do not waste cycles looking up the property id when we have the
actual property already.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170713162538.22788-3-peda@axentia.se