drm/i915: Change mutex initialization in intel_guc_log

drm/i915: Change mutex initialization in intel_guc_log

The intel_guc_log->relay.lock is currently initialized in
intel_guc_log_init_early(), but it lacks a corresponding destructor,
which can lead to a memory leak.

This patch replaces the use of mutex_init() with drmm_mutex_init(),
which ensures the lock is properly destroyed when the driver is
unloaded.

Signed-off-by: Luiz Otavio Mello <luiz.mello@estudante.ufscar.br>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250908131518.36625-4-luiz.mello@estudante.ufscar.br
Acked-by: Tvrtko Ursulin <tursulin@ursulin.net>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>