drm/xe: Name and document Wa_14019789679

Early in the development of Xe we identified an issue with SVG state
handling on DG2 and MTL (and later on Xe2 as well).  In
commit 72ac3047 ("drm/xe: Emit SVG state on RCS during driver load
on DG2 and MTL") and commit fb24b858 ("drm/xe/xe2: Update SVG state
handling") we implemented our own workaround to prevent SVG state from
leaking from context A to context B in cases where context B never
issues a specific state setting.

The hardware teams have now created official workaround Wa_14019789679
to cover this issue.  The workaround description only requires emitting
3DSTATE_MESH_CONTROL, since they believe that's the only SVG instruction
that would potentially remain unset by a context B, but still cause
notable issues if unwanted values were inherited from context A.
However since we already have a more extensive implementation that emits
the entire SVG state and prevents _any_ SVG state from unintentionally
leaking, we'll stick with our existing implementation just to be safe.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812181042.2013508-2-matthew.d.roper@intel.com