drm/i915: Fix NULL pointer dereference in capture_engine

When the intel_context structure contains NULL,
it raises a NULL pointer dereference error in drm_info().

Fixes: e8a3319c ("drm/i915: Allow error capture without a request")
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12309


Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: <stable@vger.kernel.org> # v6.3+
Signed-off-by: Eugene Kobyak <eugene.kobyak@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/xmsgfynkhycw3cf56akp4he2ffg44vuratocsysaowbsnhutzi@augnqbm777at


(cherry picked from commit 754302a5)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>