drm/kms: allocate framebuffer cmap

Without an allocated colormap, FBIOGETCMAP fails.  This would make
programs restore an all-black colormap ("links -g") or fail to work
altogether ("mplayer -vo fbdev2").

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>