If drm_modeset_lock() returns -EDEADLK, the caller is supposed to drop
all currently held locks using drm_modeset_backoff(). Failing to do so
will result in warnings and backtraces on the paths trying to lock a
contended lock. Add support for optionally printing the backtrace on the
path that hit the deadlock and didn't gracefully handle the situation.

For example, the patch [1] inadvertently dropped the return value check
and error return on replacing calc_watermark_data() with
intel_compute_global_watermarks(). The backtraces on the subsequent
locking paths hitting WARN_ON(ctx->contended) were unhelpful, but adding
the backtrace to the deadlock path produced this helpful printout:

<7> [98.002465] drm_modeset_lock attempting to lock a contended lock without backoff:
   drm_modeset_lock+0x107/0x130
   drm_atomic_get_plane_state+0x76/0x150
   skl_compute_wm+0x251d/0x2b20 [i915]
   intel_atomic_check+0x1942/0x29e0 [i915]
   drm_atomic_check_only+0x554/0x910
   drm_atomic_nonblocking_commit+0xe/0x50
   drm_mode_atomic_ioctl+0x8c2/0xab0
   drm_ioctl_kernel+0xac/0x140

Add new CONFIG_DRM_DEBUG_MODESET_LOCK to enable modeset lock debugging
with stack depot and trace.

[1] https://lore.kernel.org/r/20210924114741.15940-4-jani.nikula@intel.com

v2:
- default y if DEBUG_WW_MUTEX_SLOWPATH (Daniel)
- depends on DEBUG_KERNEL

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001091444.8177-1-jani.nikula@intel.com