drm/modeset: Prime modeset lock vs dma_resv

drm/modeset: Prime modeset lock vs dma_resv

It's kinda really hard to get this wrong on a driver with both display
and dma_resv locking. But who ever knows, so better to make sure that
really all drivers nest these the same way.

For actual lock semantics the acquire context nesting doesn't matter.
But to teach lockdep what's going on with ww_mutex the acquire ctx is
a fake lockdep lock, hence from a lockdep pov it does matter. That's
why I figured better to include it.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119210844.16947-2-daniel.vetter@ffwll.ch