drm/fb-helper: remove drm_fb_helper_save_lut_atomic

drm_fb_helper_save_lut_atomic is redundant since the .gamma_store is
now always kept up to date by drm_fb_helper_setcmap.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1499164632-5582-4-git-send-email-peda@axentia.se