drm/exec, drm/xe: Avoid abusing the drm_exec retry pointer

drm/exec, drm/xe: Avoid abusing the drm_exec retry pointer

The xe driver was using the drm_exec retry pointer directly to
restart the locking loop after out-of-memory errors. This is
relying on undocumented behaviour.

Instead add a drm_exec_retry() macro that can be used in this
situation, and that also warns if the struct drm_exec is
not newly (re-)initialized.

Use that macro in xe.

v2:
- Only allow if the drm_exec context is newly initialized.
  (Christian)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patch.msgid.link/20260520101616.41284-4-thomas.hellstrom@linux.intel.com