drm: stm: remove dead code and pointless local lut storage

The redundant fb helper .load_lut is no longer used, and can not
work right without also providing the fb helpers .gamma_set and
.gamma_get thus rendering the code in this driver suspect.

Just remove the dead code.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170713162538.22788-14-peda@axentia.se