drm/arc: Nuke event_list

This is just used for cleanup in preclose, and with the reworked event
handling code this is now done properly by the core.

Nuke it!

But it also shows that arc totally fails at sending out drm events for
flips. Next patch will hack that up.

v2: Rebase it!

Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-2-git-send-email-daniel.vetter@ffwll.ch