drm/crc: Handle opening and closing crc better

When I was doing a grep . -r /sys/kernel/debug/dri/0 I noticed a WARN
appearing when I aborted the grep with ^C.

After investigating I've also noticed that the error handling was
lacking and there are race conditions involving multiple calls to
open/close simultaneously.

Fix this by setting the opened flag first and using crc->entries to
decide when crc can be collected.

Also call unset crc source before cleaning up, this way there is
no race with a future open().

This patch has been tested with all the tests in igt with CRC in their
name.

Signed-off-by: Maarten Lankhorst <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Tomeu Vizoso <[email protected]>
[mlankhorst: Add description that this patch has been tested with IGT,
based on tomeu's feedback]

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