drm/tegra: hub: Remove bogus connection mutex check

The Tegra DRM never actually took that lock, so the driver was broken
until generic locking was added in commit:

	commit b962a120
	Author: Rob Clark <robdclark@gmail.com>
	Date:   Mon Oct 22 14:31:22 2018 +0200

	    drm/atomic: integrate modeset lock with private objects

It's now no longer necessary to take that lock, so drop the check.

v2: add rationale for why it is now safe to drop the check (Daniel)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>