drm/atmel-hlcdc: Handle drm_atomic_helper_swap_state failure

drm/atmel-hlcdc: Handle drm_atomic_helper_swap_state failure

drm_atomic_helper_swap_state() will be changed to interruptible waiting
in the next few commits, so all drivers have to be changed to handling
failure.

Atmel tracks pending commits through dc->commit.pending, so it can
ignore the changes by setting stall = false. We never return failure in
this case, so make failure a BUG_ON.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170711143314.2148-5-maarten.lankhorst@linux.intel.com
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>