drm/doc: Switch to sphinx/rst fixed-width quoting

Just fallout from switching from asciidoc to sphinx/rst.

v2: Found more. Also s/\//#/ in the vgpu ascii-art - sphinx treats
those as comments and switch to variable-width, which wreaks the
layout.

v3: Undo some of the hacks, rebasing onto latest version of Jani's
series fixed it.

Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

include/drm/drm_modes.h

@@ -169,6 +169,8 @@ enum drm_mode_status {
  *
  * The horizontal and vertical timings are defined per the following diagram.
  *
+ * ::
+ *
  *
  *               Active                 Front           Sync           Back
  *              Region                 Porch                          Porch