hw/display/tcx: add missing 64-bit access for framebuffer blitter

hw/display/tcx: add missing 64-bit access for framebuffer blitter

Commit ae5643ecc6 "hw/display/tcx: Allow 64-bit accesses to framebuffer stippler
and blitter" enabled 64-bit access for the TCX framebuffer stippler and blitter
but missed applying the change to one of the blitter MemoryRegions.

Whilst the original change works for me on my local NetBSD test image, the latest
NetBSD ISO panics on startup without this fix.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Fixes: ae5643ecc6 ("hw/display/tcx: Allow 64-bit accesses to framebuffer stippler and blitter")
Buglink: https://bugs.launchpad.net/bugs/1892540
Message-Id: <20201120081754.18250-1-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>