docs: gpu: i915.rst: fix warnings due to file renames

docs: gpu: i915.rst: fix warnings due to file renames

Fix two warnings due to file rename:

	WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -function csr support for dmc ./drivers/gpu/drm/i915/intel_csr.c' failed with return code 1
	WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -internal ./drivers/gpu/drm/i915/intel_csr.c' failed with return code 2

Fixes: 06d3ff6e7451 ("drm/i915: move intel_csr.[ch] under display/")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>