drm/i915: Allow mmio updates on all platforms, v2.

With intel_pipe_update begin/end we ensure that the mmio updates
don't run during vblank interrupt, using the hw counter we can
be sure that when current vblank count != vblank count at the time
of pipe_update_end the mmio update is complete.

This allows us to use mmio updates on all platforms, using the
update_plane call.

With Chris Wilson's patch to skip waiting for vblanks for
legacy_cursor_update this potentially leaves a small race
condition, in which update_plane can be called with a freed
crtc_state. Because of this commit acf4e84d


("drm/i915: Avoid stalling on pending flips for legacy cursor updates")
is temporarily reverted.

Changes since v1:
- Split out the flip_work rename.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-9-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>