drm/fb-helper: Remove unused gamma_size variable

The gamma_size variable has not been used since
commit 4abe3520 ("drm/kms/fb: use slow work mechanism for normal hotplug also.")

While in the area move a comment back to its code block.
They got separated by
commit d50ba256 ("drm/kms: start adding command line interface using fb.").

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326175546.18126-2-noralf@tronnes.org