drm/i915/guc, docs: Fix pdfdocs build error by removing nested grid

Nested grids in grid-table cells are not specified as proper ReST
constructs.
Commit 572f2a5c ("drm/i915/guc: Update firmware to v62.0.0")
added a couple of kerneldoc tables of the form:

  +---+-------+------------------------------------------------------+
  | 1 |  31:0 |  +------------------------------------------------+  |
  +---+-------+  |                                                |  |
  |...|       |  |  Embedded `HXG Message`_                       |  |
  +---+-------+  |                                                |  |
  | n |  31:0 |  +------------------------------------------------+  |
  +---+-------+------------------------------------------------------+

For "make htmldocs", they happen to work as one might expect,
but they are incompatible with "make latexdocs" and "make pdfdocs",
and cause the generated gpu.tex file to become incomplete and
unbuildable by xelatex.

Restore the compatibility by removing those nested grids in the tables.

Size comparison of generated gpu.tex:

                  Sphinx 2.4.4  Sphinx 4.2.0
  v5.14:               3238686       3841631
  v5.15-rc1:            376270        432729
  with this fix:       3377846       3998095

Fixes: 572f2a5c

 ("drm/i915/guc: Update firmware to v62.0.0")
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4a227569-074f-c501-58bb-d0d8f60a8ae9@gmail.com