Protect colormap add/removal with display lock

This fixes a bug where concurrent threads call XCreateColormap and
XFreeColormap corrupting a linked list where colormap structures
are stored.

Fixes: https://gitlab.freedesktop.org/xorg/lib/libx11/issues/94



v2: handle XCopyColormapAndFree also (Adam Jackson)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>