drm/ttm/tests: Let ttm_bo_test consider different ww_mutex implementation.

PREEMPT_RT has a different locking implementation for ww_mutex. The
base mutex of struct ww_mutex is declared as struct WW_MUTEX_BASE. The
latter is defined as `mutex' for non-PREEMPT_RT builds and `rt_mutex'
for PREEMPT_RT builds.

Using mutex_lock() directly on the base mutex in
ttm_bo_reserve_deadlock() leads to compile error on PREEMPT_RT.

The locking-selftest has its own defines to deal with this and it is
probably best to defines the needed one within the test program since
their usefulness is limited outside of well known selftests.

Provide ww_mutex_base_lock() which points to the correct function for
PREEMPT_RT and non-PREEMPT_RT builds.

Fixes: 995279d2 ("drm/ttm/tests: Add tests for ttm_bo functions")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240619144630.4DliKOmr@linutronix.de