drm: add drm_modeset_lock|unlock_all

This is the first step towards introducing the new modeset locking
scheme. The plan is to put helper functions into place at all the
right places step-by-step, so that the final patch to switch on the
new locking scheme doesn't need to touch every single driver.

This helper here will serve as the shotgun solutions for all places
where a more fine-grained locking isn't (yet) implemented.

v2: Fixup kerneldoc for unlock_all.

Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>