drm/i915: fix Sphinx build duplicate label warning

Fix the warning caused by enabling the autosectionlabel extension in the
kernel Sphinx build:

Documentation/gpu/i915.rst:610: WARNING: duplicate label
gpu/i915:layout, other instance in Documentation/gpu/i915.rst

The autosectionlabel extension adds labels to each section title for
cross-referencing, but forbids identical section titles in a
document. With kernel-doc, this includes sections titles in the included
kernel-doc comments.

In the warning message, Sphinx is unable to reference the labels in
their true locations in the kernel-doc comments in source. In this case,
there's "Layout" sections in both gt/intel_workarounds.c and
i915_reg.h. Rename the section in the latter to "File Layout".

Fixes: 58ad30cf

 ("docs: fix reference to core-api/namespaces.rst")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200417130109.12791-1-jani.nikula@intel.com