drm: armada: remove dead empty functions

The redundant fb helpers .gamma_set and .gamma_get are no longer used.
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/20170713162538.22788-6-peda@axentia.se