drm/i915/dg2: Maintain backward-compatible nested batch behavior

For tgl+, the per-context setting of MI_MODE[12] determines whether
the bits of a nested MI_BATCH_BUFFER_START instruction should be
interpreted in the traditional manner or whether they should
instead use a new tgl+ meaning that breaks backward compatibility, but
allows nesting into 3rd-level batchbuffers.  For previous platforms,
the hardware default for this register bit is to maintain
backward-compatible behavior unless a context intentionally opts into
the new behavior; however Xe_HPG flips the hardware default behavior.

From a SW perspective, we want to maintain the backward-compatible
behavior for userspace, so we'll apply a fake workaround to set it back
to the legacy behavior on platforms where the hardware default is to
break compatibility.  At the moment there is no Linux userspace that
utilizes third-level batchbuffers, so this will avoid userspace from
needing to make any changes.  using the legacy meaning is the correct
thing to do.  If/when we have userspace consumers that want to utilize
third-level batch nesting, we can provide a context parameter to allow
them to opt-in.

Bspec: 45974, 45718
Cc: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805163647.801064-9-matthew.d.roper@intel.com


Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>