drm: Replace drm_modeset_lock/unlock_all with DRM_MODESET_LOCK_ALL_* helpers

As suggested by the TODO list for the kernel DRM subsystem, replace
the deprecated functions that take/drop modeset locks with new helpers.

Signed-off-by: Michal Orzel <michalorzel.eng@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1588093804-30446-1-git-send-email-michalorzel.eng@gmail.com