As we use a shmemfs file to hold the context state, when not in use it
may be swapped out, such as across suspend. Since we wrote into the
shmemfs without marking the pages as dirty, the contents may be dropped
instead of being written back to swap. On re-using the shmemfs file,
such as creating a new context after resume, the contents of that file
were likely garbage and so the new context could then hang the GPU.

Simply mark the page as being written when copying into the shmemfs
file, and it the new contents will be retained across swapout.

Fixes: be1cb55a ("drm/i915/gt: Keep a no-frills swappable copy of the default context state")
Cc: Sudeep Dutt <sudeep.dutt@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Venkata Ramana Nayana <venkata.ramana.nayana@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v5.8+
Link: https://patchwork.freedesktop.org/patch/msgid/20201127120718.454037-161-matthew.auld@intel.com