fbdev: simplify fb_getput_cmap()

This function is one of the remaining users of compat_alloc_user_space()
and copy_in_user().

Clean it up by copying to a local data structure copy instead,
which also leads to more readable code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918100926.1447563-1-arnd@arndb.de