drm/i915/gt: Prevent error pointer dereference

Move the check for "if (IS_ERR(obj))" in front of the call to
i915_gem_object_set_cache_coherency() which dereferences "obj".
Otherwise it will lead to a crash.

Fixes: 43aa755e ("drm/i915/mtl: Update cache coherency setting for context structure")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/455b2279-2e08-4d00-9784-be56d8ee42e3@moroto.mountain