drm/v3d: switch to drmm_mutex_init

mutex_init is supposed to be balanced by a call to mutex_destroy, but
this is not currently happening on the v3d driver.

Considering the introduction of a DRM-managed mutex_init variant, switch
to the drmm_mutex_init.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221108175425.39819-2-mcanal@igalia.com