drm/v3d: add missing mutex_destroy

v3d_perfmon_open_file() instantiates a mutex for a particular file
instance, but it never destroys it by calling mutex_destroy() in
v3d_perfmon_close_file().

Similarly, v3d_perfmon_create_ioctl() instantiates a mutex for a
particular perfmon, but it never destroys it by calling mutex_destroy()
in v3d_perfmon_destroy_ioctl().

So, add the missing mutex_destroy on both cases.

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/20221108175425.39819-3-mcanal@igalia.com