drm/exec: Make the drm_exec_until_all_locked() macro more readable

drm/exec: Make the drm_exec_until_all_locked() macro more readable

Use __UNIQUE_ID as done elsewhere in the kernel rather than a
hand-rolled __PASTE to craft a unique id.

Also use __maybe_unused rather than (void) to signify that a
variable, althrough written to, may not actually be used.

v2:
- Move assignment to declaration (Christian)
- Declare the retry pointer as void *const.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patch.msgid.link/20260520101616.41284-3-thomas.hellstrom@linux.intel.com