drm: i915: remove dead code and pointless local lut storage

The driver stores lut values from the fbdev interface, and is able
to give them back, but does not appear to do anything with these
lut values. The generic fb helpers have replaced this function,
and may even have made the driver work for the C8 mode from the
fbdev interface. But that is untested.

Since the fb helpers .gamma_set and .gamma_get are obsolete,
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/20170713162538.22788-10-peda@axentia.se