drm: Fix potential overflow issue in event_string array

drm: Fix potential overflow issue in event_string array

When calling scnprintf() to append recovery method to event_string,
the second argument should be `sizeof(event_string) - len`, otherwise
there is a potential overflow problem.

Fixes: b7cf9f4ac1b8 ("drm: Introduce device wedged event")
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250409014633.31303-1-jiangfeng@kylinos.cn
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>