Fix XTS regression in XCopyColormapAndFree

XCopyColormapAndFree/5 threw an assertion:

520|4 5 00014017 1 2|Assertion XCopyColormapAndFree-5.(A)
520|4 5 00014017 1 3|When a colourmap argument does not name a valid colourmap,
520|4 5 00014017 1 4|then a BadColor error occurs.
520|4 5 00014017 1 5|METH: Create a bad colourmap by creating and freeing a colourmap.
520|4 5 00014017 1 6|METH: Call test function using bad colourmap as the colourmap argument.
520|4 5 00014017 1 7|METH: Verify that a BadColor error occurs.
520|4 5 00014017 1 8|unexpected signal 6 (SIGABRT) received
220|4 5 2 15:05:53|UNRESOLVED
410|4 5 1 15:05:53|IC End
510|4|system 0: Abandoning testset: caught unexpected signal 11 (SIGSEGV)

More specifically:

lt-XCopyColormapAndFree: xcb_io.c:533: _XAllocID: Assertion `ret != inval_id' failed.

This bug was introduced (by following my advice, d'oh) in:

commit 99a2cf1aa0b58391078d5d3edf0a7dab18c7745d
Author: Tapani Pälli <tapani.palli@intel.com>
Date:   Mon May 13 08:29:49 2019 +0300

    Protect colormap add/removal with display lock

In that patch we moved the call to _XcmsCopyCmapRecAndFree inside the display lock. The problem is said routine has side effects, including trying to implicitly create a colormap in some cases. Since we don't run the XID handler until SyncHandle() we would see inconsistent internal xlib state, triggering the above assert.

Fix this by dropping and re-taking the display lock before calling into XCMS.