drm/tegra: Don't use modeset_lock_crtc

Yes the help text is unhelpful, but atomic drivers should never use
this. Just grab the lock without context or anything.

Also an aside: Checking ->active like this doesn't protect against
nonblocking commits, this is rather bogus.

Cc: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-8-daniel.vetter@ffwll.ch